pgsql: Add new variable "server_version_num", which is almost the same

Log Message:
-----------
Add new variable "server_version_num", which is almost the same as
"server_version" but uses the handy PG_VERSION_NUM which allows apps to
do things like if ($version >= 80200) without having to parse apart the
value of server_version themselves.

Greg Sabino Mullane greg@turnstep.com

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.78&r2=1.79)
    pgsql/src/backend/utils/misc:
        check_guc (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/check_guc.diff?r1=1.9&r2=1.10)
        guc.c (r1.346 -> r1.347)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.346&r2=1.347)