base: Fix race in PollQueue and remove SIGALRM workaround

There is a race between enabling asynchronous IO for a file descriptor
and IO events happening on that descriptor. A SIGIO won't normally be
delivered if an event is pending when asynchronous IO is
enabled. Instead, the signal will be raised the next time there is an
event on the FD. This changeset simulates a SIGIO by setting the
async_io flag when setting up asynchronous IO for an FD. This causes
the main event loop to poll all file descriptors to check for pending
IO. As a consequence of this, the old SIGALRM hack should no longer be
needed and is therefore removed.

src/sim/async.hh

@@ -40,14 +40,12 @@
 /// @name Asynchronous event flags.
 /// To avoid races, signal handlers simply set these flags, which are
 /// then checked in the main event loop.  Defined in main.cc.
-/// @note See the PollQueue object (in pollevent.hh) for the use of async_io and async_alarm.
 //@{
 extern volatile bool async_event;       ///< Some asynchronous event has happened.
 extern volatile bool async_statdump;    ///< Async request to dump stats.
 extern volatile bool async_statreset;   ///< Async request to reset stats.
 extern volatile bool async_exit;        ///< Async request to exit simulator.
 extern volatile bool async_io;          ///< Async I/O request (SIGIO).
-extern volatile bool async_alarm;       ///< Async alarm event (SIGALRM).
 extern volatile bool async_exception;   ///< Python exception.
 //@}