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.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
create_conversion.sgml (r1.15 -> r1.16)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_conversion.sgml.diff?r1=1.15&r2=1.16)
pgsql/src/backend/commands:
conversioncmds.c (r1.17 -> r1.18)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/conversioncmds.c.diff?r1=1.17&r2=1.18)
pgsql/src/backend/utils/mb/conversion_procs:
Makefile (r1.13 -> r1.14)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/conversion_procs/Makefile.diff?r1=1.13&r2=1.14)