main-loop.c: Handle SIGINT, SIGHUP and SIGTERM synchronously

Add the termination signals SIGINT, SIGHUP and SIGTERM to the
list of signals which we handle synchronously via a signalfd.
This avoids a race condition where if we took the SIGTERM
in the middle of qemu_shutdown_requested:
    int r = shutdown_requested;
[SIGTERM here...]
    shutdown_requested = 0;

then the setting of the shutdown_requested flag by
termsig_handler() would be lost and QEMU would fail to
shut down. This was causing 'make check' to hang occasionally.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-stable@nongnu.org