maint: use STREQ/STRNEQ.

maint: use STREQ/STRNEQ.

	* doc/bison.texinfo: Space change.
	* src/system.h (STREQ, STRNEQ): New.
	* src/files.c, src/ielr.c, src/lalr.c, src/muscle-tab.c,
	* src/output.c, src/print.c, src/print_graph.c,
	* src/reader.c, src/scan-skel.l, src/tables.c,
	* src/uniqstr.c:
	Use them.
	* src/scan-gram.l: Do not use streq.h, use system.h's STREQ.
	* cfg.mk: The documentation is an exception.