dnssd.c: Fix deadlock in `cups_enum_dests()`

dnssd.c: Fix deadlock in `cups_enum_dests()`

Deadlock happens when we are about to destroy DNSSD struct by the end
of `cups_enum_dests()`. The main thread locks the mutex when the other
thread is in avahi poll callback at function `poll()` and unlocked the
mutex before - the other thread tries to lock the mutex once poll
timeout expires, but it cannot because it was locked by the main thread
and wait there. Meanwhile the main thread tries to cancel the other
thread, but the function where the other thread is not a cancellation
point, thus the cancel event is ignored and the main thread thread waits
indefinitely for the end of the other thread.

We can make the other thread asynchronous (which would cancel the thread
immediately) or release the mutex earlier in `cupsDNSSDDelete()`. The
commit does the latter.

Fixes CI for Linux