muscle: factor the handling of obsolete of obsolete directives.

muscle: factor the handling of obsolete of obsolete directives.

Suggested by Joel E. Denny.

	* src/muscle_tab.h, src/muscle_tab.c (muscle_percent_define_ensure):
	New, extracted from...
	* src/parse-gram.y (prologue_declaration: pure-parser): here.
	Remove it.
	(prologue_declaration: "%<flag>"): Use
	muscle_percent_define_ensure.
	(%error-verbose, %pure-parser): No longer tokens.
	* src/scan-gram.l (pure-parser): Return as a %<flag>.