Update mixer volume when fppd starts. Issue #65.

When parameters are extracted from command line or setting file, the function setVolume is called.
In setVolume, command "amixer set PCM xx%" is performed to apply the volume.
In fppxml.php, send to fpp the real value (0-100), and not the value for amixer (50 to 100).
Add missing headers inclusion in settings.c

Works fine in my configuration. If code seems good, can be used for fpp.
(I will keep this fix for me until we have an official one)
Stephane