Fully enforce uniqueness of constraint names.

It's been true for a long time that we expect names of table and domain
constraints to be unique among the constraints of that table or domain.
However, the enforcement of that has been pretty haphazard, and it missed
some corner cases such as creating a CHECK constraint and then an index
constraint of the same name (as per recent report from André Hänsel).
Also, due to the lack of an actual unique index enforcing this, duplicates
could be created through race conditions.

Moreover, the code that searches pg_constraint has been quite inconsistent
about how to handle duplicate names if one did occur: some places checked
and threw errors if there was more than one match, while others just
processed the first match they came to.

To fix, create a unique index on (conrelid, contypid, conname).  Since
either conrelid or contypid is zero, this will separately enforce
uniqueness of constraint names among constraints of any one table and any
one domain.  (If we ever implement SQL assertions, and put them into this
catalog, more thought might be needed.  But it'd be at least as reasonable
to put them into a new catalog; having overloaded this one catalog with
two kinds of constraints was a mistake already IMO.)  This index can replace
the existing non-unique index on conrelid, though we need to keep the one
on contypid for query performance reasons.

Having done that, we can simplify the logic in various places that either
coped with duplicates or neglected to, as well as potentially improve
lookup performance when searching for a constraint by name.

Also, as per our usual practice, install a preliminary check so that you
get something more friendly than a unique-index violation report in the
case complained of by André.  And teach ChooseIndexName to avoid choosing
autogenerated names that would draw such a failure.

While it's not possible to make such a change in the back branches,
it doesn't seem quite too late to put this into v11, so do so.

Discussion: https://postgr.es/m/0c1001d4428f$0942b430$1bc81c90$@webkr.de

doc/src/sgml/ref/create_table.sgml

@@ -2004,6 +2004,30 @@ CREATE TABLE cities_partdef
    </para>
   </refsect2>
 
+  <refsect2>
+   <title>Constraint Naming</title>
+
+   <para>
+    The SQL standard says that table and domain constraints must have names
+    that are unique across the schema containing the table or domain.
+    <productname>PostgreSQL</productname> is laxer: it only requires
+    constraint names to be unique across the constraints attached to a
+    particular table or domain.  However, this extra freedom does not exist
+    for index-based constraints (<literal>UNIQUE</literal>,
+    <literal>PRIMARY KEY</literal>, and <literal>EXCLUDE</literal>
+    constraints), because the associated index is named the same as the
+    constraint, and index names must be unique across all relations within
+    the same schema.
+   </para>
+
+   <para>
+    Currently, <productname>PostgreSQL</productname> does not record names
+    for <literal>NOT NULL</literal> constraints at all, so they are not
+    subject to the uniqueness restriction.  This might change in a future
+    release.
+   </para>
+  </refsect2>
+
   <refsect2>
    <title>Inheritance</title>