maint: avoid "magic number exit".

* cfg.mk (local-checks-to-skip): No longer skip it.
	* bootstrap.conf (gnulib_modules): Add sysexits.
	* doc/bison.texinfo, etc/bench.pl.in, src/parse-gram.y,
	* src/system.h, tests/calc.at, tests/named-refs.at: Use assert
	where appropriate instead of "if (...) exit".
	Use symbolic exit status elsewhere.

lib/.gitignore

@@ -263,3 +263,4 @@
 /closeout.h
 /fpending.c
 /fpending.h
+/sysexits.in.h