Fix volatility markings of some contrib I/O functions.

In general, datatype I/O functions are supposed to be immutable or at
worst stable.  Some contrib I/O functions were, through oversight, not
marked with any volatility property at all, which made them VOLATILE.
Since (most of) these functions actually behave immutably, the erroneous
marking isn't terribly harmful; but it can be user-visible in certain
circumstances, as per a recent bug report from Joe Van Dyk in which a
cast to text was disallowed in an expression index definition.

To fix, just adjust the declarations in the extension SQL scripts.  If we
were being very fussy about this, we'd bump the extension version numbers,
but that seems like more trouble (for both developers and users) than the
problem is worth.

A fly in the ointment is that chkpass_in actually is volatile, because
of its use of random() to generate a fresh salt when presented with a
not-yet-encrypted password.  This is bad because of the general assumption
that I/O functions aren't volatile: the consequence is that records or
arrays containing chkpass elements may have input behavior a bit different
from a bare chkpass column.  But there seems no way to fix this without
breaking existing usage patterns for chkpass, and the consequences of the
inconsistency don't seem bad enough to justify that.  So for the moment,
just document it in a comment.

Since we're not bumping version numbers, there seems no harm in
back-patching these fixes; at least future installations will get the
functions marked correctly.

contrib/chkpass/chkpass--1.0.sql

@@ -10,12 +10,15 @@
 CREATE FUNCTION chkpass_in(cstring)
 	RETURNS chkpass
 	AS 'MODULE_PATHNAME'
-	LANGUAGE C STRICT;
+	LANGUAGE C STRICT VOLATILE;
+-- Note: chkpass_in actually is volatile, because of its use of random().
+-- In hindsight that was a bad idea, but there's no way to change it without
+-- breaking some usage patterns.
 
 CREATE FUNCTION chkpass_out(chkpass)
 	RETURNS cstring
 	AS 'MODULE_PATHNAME'
-	LANGUAGE C STRICT;
+	LANGUAGE C STRICT IMMUTABLE;
 
 CREATE TYPE chkpass (
 	internallength = 16,

contrib/ltree/ltree--1.0.sql

@@ -6,12 +6,12 @@
 CREATE FUNCTION ltree_in(cstring)
 RETURNS ltree
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE FUNCTION ltree_out(ltree)
 RETURNS cstring
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE TYPE ltree (
 	INTERNALLENGTH = -1,
@@ -303,12 +303,12 @@ CREATE OPERATOR CLASS ltree_ops
 CREATE FUNCTION lquery_in(cstring)
 RETURNS lquery
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE FUNCTION lquery_out(lquery)
 RETURNS cstring
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE TYPE lquery (
 	INTERNALLENGTH = -1,
@@ -414,12 +414,12 @@ CREATE OPERATOR ^? (
 CREATE FUNCTION ltxtq_in(cstring)
 RETURNS ltxtquery
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE FUNCTION ltxtq_out(ltxtquery)
 RETURNS cstring
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE TYPE ltxtquery (
 	INTERNALLENGTH = -1,
@@ -481,12 +481,12 @@ CREATE OPERATOR ^@ (
 CREATE FUNCTION ltree_gist_in(cstring)
 RETURNS ltree_gist
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE FUNCTION ltree_gist_out(ltree_gist)
 RETURNS cstring
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE TYPE ltree_gist (
 	internallength = -1,

contrib/pg_trgm/pg_trgm--1.1.sql

@@ -53,12 +53,12 @@ CREATE OPERATOR <-> (
 CREATE FUNCTION gtrgm_in(cstring)
 RETURNS gtrgm
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE FUNCTION gtrgm_out(gtrgm)
 RETURNS cstring
 AS 'MODULE_PATHNAME'
-LANGUAGE C STRICT;
+LANGUAGE C STRICT IMMUTABLE;
 
 CREATE TYPE gtrgm (
         INTERNALLENGTH = -1,