Fixed connection loss / failsafe detection, added decoding of two switches

#111 px4io integrated mixing

Fix a botched merge for #106 at the same time.

Initial implementation of application access to the PX4IO relays.

Add FIONWRITE to allow applications to sniff the amount of writable space on a descriptor. Implement this for serial devices only.

Fixed connection loss / failsafe detection, added decoding of two switch channels if IO supports 18 RC channels. Loss is tested, switch channels are not.

Rework the way we scan text for scaler definitions; something weird was going on with sscanf's handling of %n that wasn't obvious. This seems to work around the issue.

This reverts commit 668d1b33.

Make mixer ioctls load from a memory buffer rather than a file. This is prep for uploading the memory buffer to IO to be processed there.

Updated PX4IOAR start script

Untangle the DSM decoder from the input source priority logic, clean up input handling some more.

Made MPU-6000 gyro optional

Don't call ::close on ORB publication handles.

Fix several aspects of the PWM output driver; enable auto-reload, use named constants for various control bits, and use a more polite mechanism at disarm time to avoid runt pulses.

This may address an issue we've seen where we get occasional malformed PWM output pulses, possibly due to a race between compare updates and the timer.

Whitespace.