Allow non-superuser database owners to create procedural languages.

Allow non-superuser database owners to create procedural languages.
A DBA is allowed to create a language in his database if it's marked
"tmpldbacreate" in pg_pltemplate.  The factory default is that this is set
for all standard trusted languages, but of course a superuser may adjust
the settings.  In service of this, add the long-foreseen owner column to
pg_language; renaming, dropping, and altering owner of a PL now follow
normal ownership rules instead of being superuser-only.
Jeremy Drake, with some editorialization by Tom Lane.