Hurd, C++: Avoid "const char *" to "char *" casts

Hurd, C++: Avoid "const char *" to "char *" casts

... by a bit of code refactoring:

	gdb/
	* gnu-nat.c (set_task_pause_cmd, set_signals_cmd)
	(set_exceptions_cmd): Add variants taking an "int arg" instead of
	a "char *".  Make the "char *" variants use the former.
	(set_noninvasive_cmd): Also use the "int arg" variants.