pgsql: Add a new ereport auxiliary function errdetail_log(), which works

Log Message:
-----------
Add a new ereport auxiliary function errdetail_log(), which works the same as
errdetail except the string goes only to the server log, replacing the normal
errdetail there. This provides a reasonably clean way of dealing with error
details that are too security-sensitive or too bulky to send to the client.

This commit just adds the infrastructure --- actual uses to follow.

Modified Files:
--------------
pgsql/doc/src/sgml:
sources.sgml (r2.29 -> r2.30)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/sources.sgml?r1=2.29&r2=2.30)
pgsql/src/backend:
nls.mk (r1.21 -> r1.22)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nls.mk?r1=1.21&r2=1.22)
pgsql/src/backend/port:
ipc_test.c (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/ipc_test.c?r1=1.23&r2=1.24)
pgsql/src/backend/utils/error:
elog.c (r1.202 -> r1.203)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c?r1=1.202&r2=1.203)
pgsql/src/include/utils:
elog.h (r1.91 -> r1.92)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/elog.h?r1=1.91&r2=1.92)