When disabling target async, remove all target event sources from the event loop

The sigall-reverse.exp test occasionally fails with something like this:

 (gdb) PASS: gdb.reverse/sigall-reverse.exp: send signal TERM
 continue
 Continuing.
 The next instruction is syscall exit_group.  It will make the program exit.  Do you want to stop the program?([y] or n) FAIL: gdb.reverse/sigall-reverse.exp: continue to signal exit (timeout)
 FAIL: gdb.reverse/sigall-reverse.exp: reverse to handler of TERM (timeout)
 FAIL: gdb.reverse/sigall-reverse.exp: reverse to gen_TERM (timeout)

This is another event-loop/async related problem exposed by the patch
that made 'query' use gdb_readline_wrapper (588dcc3edbde19f9).

The problem is that even though gdb_readline_wrapper disables
target-async while the secondary prompt is in progress, the record
target's async event source is left marked.  So when
gdb_readline_wrapper nests an event loop to process input, it may
happen that that event loop ends up processing a target event while
GDB is not really ready for it.  Here's the relevant part of the
backtrace showing the root issue in action:

...
 #14 0x000000000061cb48 in fetch_inferior_event (client_data=0x0) at src/gdb/infrun.c:4158
 #15 0x0000000000642917 in inferior_event_handler (event_type=INF_REG_EVENT, client_data=0x0) at src/gdb/inf-loop.c:57
 #16 0x000000000077ca5c in record_full_async_inferior_event_handler (data=0x0) at src/gdb/record-full.c:791
 #17 0x0000000000640fdf in invoke_async_event_handler (data=...) at src/gdb/event-loop.c:1067
 #18 0x000000000063fb01 in process_event () at src/gdb/event-loop.c:339
 #19 0x000000000063fb2a in gdb_do_one_event () at src/gdb/event-loop.c:360
 #20 0x000000000074d607 in gdb_readline_wrapper (prompt=0x3588f40 "The next instruction is syscall exit_group.  It will make the program exit.  Do you want to stop the program?([y] or n) ") at src/gdb/top.c:842
 #21 0x0000000000750bd9 in defaulted_query (ctlstr=0x8c6588 "The next instruction is syscall exit_group.  It will make the program exit.  Do you want to stop the program?", defchar=121 'y', args=0x7fff70524410) at src/gdb/utils.c:1279
 #22 0x0000000000750e4c in yquery (ctlstr=0x8c6588 "The next instruction is syscall exit_group.  It will make the program exit.  Do you want to stop the program?") at src/gdb/utils.c:1358
 #23 0x00000000004b020e in record_linux_system_call (syscall=gdb_sys_exit_group, regcache=0x3529450, tdep=0xd6c840 <amd64_linux_record_tdep>) at src/gdb/linux-record.c:1933

With my all-stop-on-top-of-non-stop series, I'm also seeing
gdb.server/ext-attach.exp fail occasionally due to the same issue.

The first part of the fix is for target_async implementations to make
sure to remove/unmark all target-related event sources from the event
loop.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/
2015-02-03  Pedro Alves  <palves@redhat.com>

	* event-loop.c (clear_async_event_handler): New function.
	* event-loop.h (clear_async_event_handler): New declaration.
	* record-btrace.c (record_btrace_async): New function.
	(init_record_btrace_ops): Install record_btrace_async.
	* record-full.c (record_full_async): New function.
	(record_full_resume): Don't mark the async event source here.
	(init_record_full_ops): Install record_full_async.
	(record_full_core_resume): Don't mark the async event source here.
	(init_record_full_core_ops): Install record_full_async.
	* remote.c (remote_async): Mark and clear the async stop reply
	queue event-loop token as appropriate.

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