Remove configure probes for symlink/readlink, and dead code.

symlink() and readlink() are in SUSv2 and all targeted Unix systems have
them.  We have partial emulation on Windows.  Code that raised runtime
errors on systems without it has been dead for years, so we can remove
that and also references to such systems in the documentation.

Define HAVE_READLINK and HAVE_SYMLINK macros on Unix.  Our Windows
replacement functions based on junction points can't be used for
relative paths or for non-directories, so the macros can be used to
check for full symlink support.  The places that deal with tablespaces
can just use symlink functions without checking the macros.  (If they
did check the macros, they'd need to provide an #else branch with a
runtime or compile time error, and it'd be dead code.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

doc/src/sgml/manage-ag.sgml

@@ -530,13 +530,6 @@ SELECT spcname FROM pg_tablespace;
    is also useful for listing the existing tablespaces.
   </para>
 
-  <para>
-   <productname>PostgreSQL</productname> makes use of symbolic links
-   to simplify the implementation of tablespaces. This
-   means that tablespaces can be used <emphasis>only</emphasis> on systems
-   that support symbolic links.
-  </para>
-
   <para>
    The directory <filename>$PGDATA/pg_tblspc</filename> contains symbolic links that
    point to each of the non-built-in tablespaces defined in the cluster.

doc/src/sgml/ref/create_tablespace.sgml

@@ -127,10 +127,6 @@ CREATE TABLESPACE <replaceable class="parameter">tablespace_name</replaceable>
  <refsect1>
   <title>Notes</title>
 
-  <para>
-   Tablespaces are only supported on systems that support symbolic links.
-  </para>
-
    <para>
     <command>CREATE TABLESPACE</command> cannot be executed inside a transaction
     block.