pgsql: Refactor some lsyscache routines to eliminate duplicate code and

Log Message:
-----------
Refactor some lsyscache routines to eliminate duplicate code and save
a couple of syscache lookups in make_pathkey_from_sortinfo().

Modified Files:
--------------
pgsql/src/backend/optimizer/path:
pathkeys.c (r1.82 -> r1.83)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/pathkeys.c.diff?r1=1.82&r2=1.83)
pgsql/src/backend/utils/cache:
lsyscache.c (r1.144 -> r1.145)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/lsyscache.c.diff?r1=1.144&r2=1.145)
pgsql/src/include/utils:
lsyscache.h (r1.113 -> r1.114)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/lsyscache.h.diff?r1=1.113&r2=1.114)