Modify fmgr so that internal name (compiler name) of a built-in

Modify fmgr so that internal name (compiler name) of a built-in
function is found in prosrc field of pg_proc, not proname.  This allows
multiple aliases of a built-in to all be implemented as direct builtins,
without needing a level of indirection through an SQL function.  Replace
existing SQL alias functions with builtin entries accordingly.
Save a few K by not storing string names of builtin functions in fmgr's
internal table (if you really want 'em, get 'em from pg_proc...).
Update opr_sanity with a few more cross-checks.