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