Have SIGTERM promptly quit GDB even when the dummy target is active

Have SIGTERM promptly quit GDB even when the dummy target is active

GDB currently does not promptly quit after receiving a SIGTERM while no
proper target is active.  This is because in handle_sigterm we currently
look at target_can_async_p to determine whether to asynchronously quit
GDB using an async signal handler or to asynchronously quit using the
quit flag.  However, target_can_async_p is always false under the dummy
target, so under this target we always use the quit flag and not the
async signal handler to signal that GDB should quit.  So GDB won't quit
until a code path that checks the quit flag is executed.

To fix this issue, this patch makes the SIGTERM handler no longer
inspect target_can_async_p, and instead makes the handler
unconditionally set the quit flag _and_ mark the corresponding async
signal handler, so that if the target is async (or if it's the dummy
target) then we will likely quit through the async signal handler, and
if it's not async then we will likely quit through the quit flag.  This
redundant approach is similar to how we handle SIGINT.

gdb/ChangeLog:

	* event-top.c (handle_sigterm): Don't inspect
	target_can_async_p.  Always set the quit flag and always mark
	the async signal handler.

gdb/testsuite/ChangeLog:

	* gdb.base/gdb-sigterm-2.exp: New test.