Some users want certain clients to fully control playback, but do not
want them to be able to trigger database update.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1124

From the feature request: "I generally like to have crossfade on, but
when it happens during such short tracks (e.g. 20 seconds or less) it
doesn't really sound good as those tracks are not really meant to be
crossfaded and intended to act as a bridge on their own."

Sounds reasonable.  This commit doesn't add an option, but hard-codes
the limit to 20 seconds.  If it turns out that users want to have it
configurable, we can still add the option.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1184

This reduces duplicates in the music database by hiding the original
song file when it is referenced by a CUE sheet.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1275

Don't allocate any memory until the output is really enabled.

The implementation was removed 12 years ago in commit f6455d5f and
nobody missed it.

This reverts commit 9200fa6d.  It was
wrong because it works only if the target song has already been
scanned.

Allows win32_main() to throw exceptions.

These malformed strings must not be transferred over the wire, because
the MPD protocol is defined to be UTF-8.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1274

The wrong variable was used.  This has been broken by commit
4eeea640 seven years ago - and nobody as noticed, d'oh!

libavahi-client uses this one to schedule events immediately.
However, CoarseTimerEvent may be called too late, and cause timeouts.