Introduce "builtin" collation provider.

New provider for collations, like "libc" or "icu", but without any
external dependency.

Initially, the only locale supported by the builtin provider is "C",
which is identical to the libc provider's "C" locale. The libc
provider's "C" locale has always been treated as a special case that
uses an internal implementation, without using libc at all -- so the
new builtin provider uses the same implementation.

The builtin provider's locale is independent of the server environment
variables LC_COLLATE and LC_CTYPE. Using the builtin provider, the
database collation locale can be "C" while LC_COLLATE and LC_CTYPE are
set to "en_US", which is impossible with the libc provider.

By offering a new builtin provider, it clarifies that the semantics of
a collation using this provider will never depend on libc, and makes
it easier to document the behavior.

Discussion: https://postgr.es/m/ab925f69-5f9d-f85e-b87c-bd2a44798659@joeconway.com
Discussion: https://postgr.es/m/dd9261f4-7a98-4565-93ec-336c1c110d90@manitou-mail.org
Discussion: https://postgr.es/m/ff4c2f2f9c8fc7ca27c1c24ae37ecaeaeaff6b53.camel%40j-davis.com
Reviewed-by: Daniel Vérité, Peter Eisentraut, Jeremy Schneider

doc/src/sgml/ref/create_collation.sgml

@@ -96,6 +96,11 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
        <replaceable>locale</replaceable>, you cannot specify either of those
        parameters.
       </para>
+      <para>
+       If <replaceable>provider</replaceable> is <literal>builtin</literal>,
+       then <replaceable>locale</replaceable> must be specified and set to
+       <literal>C</literal>.
+      </para>
      </listitem>
     </varlistentry>
 
@@ -129,9 +134,9 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
      <listitem>
       <para>
        Specifies the provider to use for locale services associated with this
-       collation.  Possible values are
-       <literal>icu</literal><indexterm><primary>ICU</primary></indexterm>
-       (if the server was built with ICU support) or <literal>libc</literal>.
+       collation.  Possible values are <literal>builtin</literal>,
+       <literal>icu</literal><indexterm><primary>ICU</primary></indexterm> (if
+       the server was built with ICU support) or <literal>libc</literal>.
        <literal>libc</literal> is the default.  See <xref
        linkend="locale-providers"/> for details.
       </para>

doc/src/sgml/ref/create_database.sgml

@@ -162,6 +162,11 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
         linkend="create-database-lc-ctype"/>, or <xref
         linkend="create-database-icu-locale"/> individually.
        </para>
+       <para>
+        If <xref linkend="create-database-locale-provider"/> is
+        <literal>builtin</literal>, then <replaceable>locale</replaceable>
+        must be specified and set to <literal>C</literal>.
+       </para>
        <tip>
         <para>
          The other locale settings <xref linkend="guc-lc-messages"/>, <xref
@@ -243,7 +248,7 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
       <listitem>
        <para>
         Specifies the provider to use for the default collation in this
-        database.  Possible values are
+        database.  Possible values are <literal>builtin</literal>,
         <literal>icu</literal><indexterm><primary>ICU</primary></indexterm>
         (if the server was built with ICU support) or <literal>libc</literal>.
         By default, the provider is the same as that of the <xref

doc/src/sgml/ref/createdb.sgml

@@ -171,7 +171,7 @@ PostgreSQL documentation
      </varlistentry>
 
      <varlistentry>
-      <term><option>--locale-provider={<literal>libc</literal>|<literal>icu</literal>}</option></term>
+      <term><option>--locale-provider={<literal>builtin</literal>|<literal>libc</literal>|<literal>icu</literal>}</option></term>
       <listitem>
        <para>
         Specifies the locale provider for the database's default collation.

doc/src/sgml/ref/initdb.sgml

@@ -286,6 +286,11 @@ PostgreSQL documentation
         environment that <command>initdb</command> runs in. Locale
         support is described in <xref linkend="locale"/>.
        </para>
+       <para>
+        If <option>--locale-provider</option> is <literal>builtin</literal>,
+        <option>--locale</option> must be specified and set to
+        <literal>C</literal>.
+       </para>
       </listitem>
      </varlistentry>
 
@@ -314,8 +319,18 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry id="app-initdb-builtin-locale">
+      <term><option>--builtin-locale=<replaceable>locale</replaceable></option></term>
+      <listitem>
+       <para>
+        Specifies the locale name when the builtin provider is used. Locale support
+        is described in <xref linkend="locale"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="app-initdb-option-locale-provider">
-      <term><option>--locale-provider={<literal>libc</literal>|<literal>icu</literal>}</option></term>
+      <term><option>--locale-provider={<literal>builtin</literal>|<literal>libc</literal>|<literal>icu</literal>}</option></term>
       <listitem>
        <para>
         This option sets the locale provider for databases created in the new