pgsql: Alter the signature for encoding conversion functions to declare

Log Message:
-----------
Alter the signature for encoding conversion functions to declare the
output area as INTERNAL not CSTRING.  This is to prevent people from
calling the functions by hand.  This is a permanent solution for the
back branches but I hope it is just a stopgap for HEAD.

Tags:
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REL7_3_STABLE

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        create_conversion.sgml (r1.5 -> r1.5.2.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_conversion.sgml.diff?r1=1.5&r2=1.5.2.1)
    pgsql/src/backend/commands:
        conversioncmds.c (r1.5 -> r1.5.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/conversioncmds.c.diff?r1=1.5&r2=1.5.2.1)
    pgsql/src/backend/utils/mb/conversion_procs:
        Makefile (r1.7.2.1 -> r1.7.2.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/Makefile.diff?r1=1.7.2.1&r2=1.7.2.2)