demangle.h (demangler_engine): Const-ify.

include:
	* demangle.h (demangler_engine): Const-ify.
	* libiberty.h (buildargv): Likewise.

libiberty:
	* argv.c (buildargv, tests, main): Const-ify.
	* cp-demangle.c (operator_code): Likewise.
	* cplus-dem.c (optable, libiberty_demanglers,
	cplus_demangle_set_style, cplus_demangle_name_to_style,
	print_demangler_list): Likewise.
	* hashtab.c (higher_prime_number): Likewise.
	* strcasecmp.c (charmap): Likewise.
	* strerror.c (error_info, strerror, main): Likewise.
	* strncasecmp.c (charmap): Likewise.
	* strsignal.c (signal_info): Likewise.

From-SVN: r46060