fdm: don't free FD data that may be referenced by epoll return data

It is perfectly possible, and legal, for a FDM handler to delete
another handler. The problem however is when the epoll returned array
of FD events contain the removed FD handler *after* the handler that
removed it.

That is, if the epoll returned array is:

  [FD=13, FD=37]

and the handler for FD=13 removes FD=37, then given the current
implementation, the FD user data (our handler callback etc) will point
to a free:d address.

Add support for this situation by deferring FD handler removal *when
called from another handler*.

This is done by "locking" the FDM struct before looping the handlers
for FDs with events (and unlocking afterwards).

In fdm_del(), we check if the FDM has been locked, in which case the
FD is marked as deleted, and put on a deferred list. But
not *actually* deleted.

Meanwhile, in the FDM poll function, we skip calling handlers for
marked-for-deletion FDs.

Then, when all FDs have been processed, we loop the deferred list and
finally deletes the FDs for real.

fdm.h

@@ -11,5 +11,6 @@ void fdm_destroy(struct fdm *fdm);
 
 bool fdm_add(struct fdm *fdm, int fd, int events, fdm_handler_t handler, void *data);
 bool fdm_del(struct fdm *fdm, int fd);
+bool fdm_del_no_close(struct fdm *fdm, int fd);
 
 bool fdm_poll(struct fdm *fdm);