[RFAv2] Implement show | set may-call-functions [on|off]

[Eli Zaretskii]

> From: Philippe Waroquiers <philippe.waroquiers@skynet.be>
> Cc: Philippe Waroquiers <philippe.waroquiers@skynet.be>
> Date: Fri, 26 Apr 2019 23:34:50 +0200
> 
> gdb/ChangeLog
> 	* NEWS: Mention the new set|show may-call-functions.
> 	* infcall.c (may_call_functions_p): New variable.
> 	(show_may_call_functions_p): New function.
> 	(call_function_by_hand_dummy): Throws an error if not
> 	may-call-functions.
> 	(_initialize_infcall): Call add_setshow_boolean_cmd for
> 	may-call-functions.
> 
> gdb/testsuite/ChangeLog
> 	* gdb.base/callexit.exp: Test may-call-functions off.
> 
> gdb/doc/ChangeLog
> 	* gdb.texinfo (Calling): Document the new
> 	set|show may-call-functions.

OK for the documentation parts, thanks.