Fixes use-after-free bug (https://bugs.musicpd.org/view.php?id=4548).

Comes with a callback pointer instead of a virtual method, which
allows multiple instances in one class.

Replaces the old BoundMethod template.

bind fails with an "address already in use" error if the socket path
already exists.

This was broken by cbb595ba.

Manages memory automatically.

The implicit cast operator can be very dangerous.

Must return -1 on error, not 0.  0 is root.

The previous commit was broken.  D'oh!

The IsActive() method returned true even if the timer was not active,
after it completed once.  This broke the state file timer, and the
state file was not saved periodically.

This was broken by commit 84d20d9e, which deleted the "!" from the
check.

This was broken by commit 84d20d9e, which deleted the "!" from the
check.

Turns out the lock-free code using atomics was not thread-safe.  The
given callback could be invoked by GLib before the source_id attribute
was assigned.  This commit changes the DeferredMonitor class to use a
Mutex to block the event loop until source_id is assigned.  This bug
does not exist in the 0.19 branch because it does not use the GLib
main loop anymore.

Make sure glibc gives us all features.