Change tsearch2 to not use the unsafe practice of creating functions

Change tsearch2 to not use the unsafe practice of creating functions
that return INTERNAL without also having INTERNAL arguments.  Since the
functions in question aren't meant to be called by hand anyway, I just
redeclared them to take 'internal' instead of 'text'.  Also add code
to ProcedureCreate() to enforce the restriction, as I should have done
to start with :-(