Add support to write pid to pidfile when running in daemon mode.
Move daemon related code to a separate file.
Remove pid file when exiting.
Support log redirection to a file.
Use line buffering on log files.
This makes sure the contents are written on the disk, so the user can
read the log.
Tiny cleanup for merge
shairport: do not try and deinit output if it hasn't been set yet
check supplied AP name length to comply with mDNS spec.
Alsa output driver

Based on a bit of code of @Skaman and bit of mine, I've added support
for mixer and audio Alsa support.
README.md: updates
TODO updates (nearly there!)
configure: fix message
audio_ao, audio_pulse: make usage indent consistent
Makefile: force rebuild after reconfigure
configure: clean up pkg-config code into a function