system/ByteOrder: <endian.h> is a non-standard header that only Linux provides.
event/TimeoutMonitor: reset "active" flag before invoking OnTimeout()

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.
Merge branch 'v0.18.x'
event/TimeoutMonitor: really reset "active" flag before invoking OnTimeout()

The previous commit was broken.  D'oh!
Merge branch 'v0.18.x'
doc/user: more markup
doc/user: add comment about Linux and OSS
doc/user: add comments about libao and openal
doc/user: document the state file
StateFile: make "path" const
StateFile: configurable interval
doc/{mpd.conf.5,mpdconf.example}: move documentation to the manual
doc/mpd.conf.5: remove deprecated setting
decoder/mad: make variables more local
decoder/mad: convert enums/macros to constexpr
Merge remote-tracking branch 'upstream/master'