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

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

Inferior function calls are powerful but might lead to undesired
results such as crashes when calling nested functions (frequently
used in particular in Ada).

This implements a GDB setting to disable calling inferior functions.

Note: the idea is that if/when the 'slash command' patch is pushed,
that this setting can be changed e.g. by using the shortcut /c.

This is version 2 of the patch.  It handles all the received comments,
mostly replace 'can-call' by 'may-call', and avoid using
'inferior function call' in factor of 'calling function in the program'.

2019-04-26  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

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.