fdm: add support for managing signals

Add fdm_signal_add() and fdm_signal_del(). Signals added to the fdm
will be monitored, and the provided callback called as “soon as
possible” from the main context (i.e not from the signal handler
context).

Monitored signals are *blocked* by default. We use epoll_pwait() to
unblock them while we’re polling. This allows us to do race-free
signal detection.

We use a single handler for all monitored signals; the handler simply
updates the signal’s slot in a global array (sized to fit SIGRTMAX
signals).

When epoll_pwait() returns EINTR, we loop the global array. The
callback associated with each signal that fired is called.

pgo/pgo.c

@@ -35,7 +35,7 @@ async_write(int fd, const void *data, size_t len, size_t *idx)
 }
 
 bool
-fdm_add(struct fdm *fdm, int fd, int events, fdm_handler_t handler, void *data)
+fdm_add(struct fdm *fdm, int fd, int events, fdm_fd_handler_t handler, void *data)
 {
     return true;
 }