This can happen if a stream is used blocking exclusively (the FD is
never registered with watcher, but is removed in the stream's destructor
just in case it ever was - doing this conditionally would require an
additional flag in streams).  There may be no thread reading from
the read end of the notify pipe (e.g. in starter), causing the write
to the notify pipe to block after it's full.  Anyway, doing a relatively
expensive FD update is unnecessary if there were no changes.

Fixes #1453.

This avoids the evaluation of %N even if the thread pool is never used.
We need to avoid as many custom printf specifiers as possible when
fuzzing our code to avoid excessive log messages.

Since the FD set could get rebuilt quite often this change avoids having
to allocate memory just to enumerate the registered FDs.

With LinuxThreads, poll() is unfortunately no cancellation point. It seems
that poll gets woken up after cancellation, but we actively must check
for cancellation before re-entering poll to properly shut down the watcher
thread.

References #840.

While they usually are not included in a normal strongSwan build, the XPC
header indirectly defines these Mach types. To build charon-xpc, which uses
both XPC and strongSwan includes, we have to redefine these types.

poll() may return POLLHUP or POLLNVAL for given file descriptors. To handle
these properly, we signal them to the EXCEPT watcher state, if registered. If
not, we call the read/write callbacks, so they can properly fail when trying
to read from or write to the file descriptor.