gdb: add "set startup-quietly" command

gdb: add "set startup-quietly" command

This adds a new command to change GDB to behave as though "-quiet"
were always given.  This new command can be added to the gdbearlyinit
file to affect future GDB sessions.

gdb/ChangeLog:

	* NEWS: Add entry.
	* main.c (captured_main_1): Call check_quiet_mode.
	* top.c (startup_quiet): New global.
	(check_quiet_mode): New function.
	(show_startup_quiet): New function.
	(init_main): Register new command.
	* top.h (check_quiet_mode): Declare.

gdb/doc/ChangeLog:

	* gdb.texinfo (Mode Options): Mention "set startup-quietly".

gdb/testsuite/ChangeLog:

	* gdb.base/startup-file.exp: Add more tests.