Alter plpgsql's lexer so that yylineno and yymore are not used. This

Alter plpgsql's lexer so that yylineno and yymore are not used.  This
avoids 'input buffer overflow' failure on long literals, improves
performance, gives the right answer for line position in functions
containing multiline literals, suppresses annoying compiler warnings,
and generally is so much better I wonder why we didn't do it before.