assignment of `mnt->mounted_database` in `SimpleDatabase::Mount`.

This fixes spurious replay gain logs when the player inserts silence
chunks, because those silence chunks had no replay gain attached,
resetting the ReplayGainFilter state, flipping it forth and back.

This attribute is not particularly important, but it was
uninitialized.

Fixes #165.

When starting playback with a specific song which does not have the
highest priority, the previous highest priority song was skipped
completely because its order was "swapped".  This commit changes to a
more expensive operation which inserts the selected song into the
order list.

This fixes a small part of #165

libupnp 1.6.24 added a few badly designed macros which break the MPD
build:

 https://sourceforge.net/p/pupnp/code/ci/8177a4195a4d4a5d6cb095eaf4ca5b65ac446381/

To work around this, we disable our emulation functions (from
714011c8) on this libupnp version.

Closes #163

That '!' accidently got lost in commit
75582d47

D'oh!

Closes #162

Signed-off-by: FlashSystems <developer@flashsystems.de>

release v0.20.12

Closes #155

Fixes a deadlock caused by commit
31ab78ae.  That commit was not
actually bad - just these two calls have always been bad, which went
unnoticed for a long time.

Avoid another potential deadlock: if no file descriptors are
registered, our non-blocking ALSA code cannot ever work.

After UnlockActivate() returns, we not only need to check for errors,
but also for more room in the ring buffer.  If we don't check the ring
buffer, it may be drained already, and the cond.wait() call will never
finish.

Closes #151

Bugs found by Coverity.