Don't rely on immediate_quit in command_line_input

Don't rely on immediate_quit in command_line_input

AFAICS, immediate_quit was only needed here nowdays to be able to
interrupt gdb_readline_no_editing.

command_line_input can also take the gdb_readline_wrapper path, but
since that is built on top of the event loop (gdb_select / poll and
asynchronous signal handlers), it can be interrupted.

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* top.c: Include "gdb_select.h".
	(gdb_readline_no_editing): Wait for input with gdb_select instead
	of blocking in fgetc.
	(command_line_input): Don't set immediate_quit.