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.

cfg.mk

@@ -43,7 +43,6 @@ local-checks-to-skip =			\
   sc_prohibit_atoi_atof			\
   sc_prohibit_doubled_word		\
   sc_prohibit_empty_lines_at_EOF	\
-  sc_prohibit_magic_number_exit		\
   sc_prohibit_strcmp
 
 # The local directory containing the checked-out copy of gnulib used in