Implement a preliminary 'template' facility for procedural languages,

Implement a preliminary 'template' facility for procedural languages,
as per my recent proposal.  For now the template data is hard-wired in
proclang.c --- this should be replaced later by a new shared system
catalog, but we don't want to force initdb during 8.1 beta.  This change
lets us cleanly load existing dump files even if they contain outright
wrong information about a PL's support functions, such as a wrong path
to the shared library or a missing validator function.  Also, we can
revert the recent kluges to make pg_dump dump PL support functions that
are stored in pg_catalog.
While at it, I removed the code in pg_regress that replaced $libdir
with a hardcoded path for temporary installations.  This is no longer
needed given our support for relocatable installations.