Don't set immediate_quit in prompt_for_continue

Don't set immediate_quit in prompt_for_continue

immediate_quit used to be necessary back when prompt_for_continue used
blocking fread, but nowadays it uses gdb_readline_wrapper, which is
implemented in terms of a nested event loop, which already knows how
to react to SIGINT:

 #0  throw_it (reason=RETURN_QUIT, error=GDB_NO_ERROR, fmt=0x9d6d7e "Quit", ap=0x7fffffffcb88)
     at .../src/gdb/common/common-exceptions.c:324
 #1  0x00000000007bab5d in throw_vquit (fmt=0x9d6d7e "Quit", ap=0x7fffffffcb88) at .../src/gdb/common/common-exceptions.c:366
 #2  0x00000000007bac9f in throw_quit (fmt=0x9d6d7e "Quit") at .../src/gdb/common/common-exceptions.c:385
 #3  0x0000000000773a2d in quit () at .../src/gdb/utils.c:1039
 #4  0x000000000065d81b in async_request_quit (arg=0x0) at .../src/gdb/event-top.c:893
 #5  0x000000000065c27b in invoke_async_signal_handlers () at .../src/gdb/event-loop.c:949
 #6  0x000000000065aeef in gdb_do_one_event () at .../src/gdb/event-loop.c:280
 #7  0x0000000000770838 in gdb_readline_wrapper (prompt=0x7fffffffcd40 "---Type <return> to continue, or q <return> to quit---")
     at .../src/gdb/top.c:873

The need for the QUIT in stdin_event_handler is then exposed by the
gdb.base/double-prompt-target-event-error.exp test, which has:

	# We're now stopped in a pagination query while handling a
	# target event (printing where the program stopped).  Quitting
	# the pagination should result in only one prompt being
	# output.
	send_gdb "\003p 1\n"

Without that change we'd get:

 Continuing.
 ---Type <return> to continue, or q <return> to quit---PASS: gdb.base/double-prompt-target-event-error.exp: ctrlc target event: continue: continue to pagination
 ^CpQuit
 (gdb)  1
 Undefined command: "1".  Try "help".
 (gdb) PASS: gdb.base/double-prompt-target-event-error.exp: ctrlc target event: continue: first prompt
 ERROR: Undefined command "".
 UNRESOLVED: gdb.base/double-prompt-target-event-error.exp: ctrlc target event: continue: no double prompt

Vs:

 Continuing.
 ---Type <return> to continue, or q <return> to quit---PASS: gdb.base/double-prompt-target-event-error.exp: ctrlc target event: continue: continue to pagination
 ^CQuit
 (gdb) p 1
 $1 = 1
 (gdb) PASS: gdb.base/double-prompt-target-event-error.exp: ctrlc target event: continue: first prompt
 PASS: gdb.base/double-prompt-target-event-error.exp: ctrlc target event: continue: no double prompt

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

	* event-top.c (stdin_event_handler): Call QUIT;
	(prompt_for_continue): Don't run with immediate_quit set.