Make LANCOMPILER clause in CREATE LANGUAGE optional. Allow "identifier"

Make LANCOMPILER clause in CREATE LANGUAGE optional.  Allow "identifier"
syntax for language names (instead of 'string').

createlang now handles the case where a second language uses the same call
handler as an already installed language (e.g., plperl/plperlu).

droplang now handles the reverse case, i.e., dropping a language where
the call handler is still used by another language.  Moreover, droplang
can now be used to drop any user-defined language, not just the supplied
ones.