Initial implementation of application access to the PX4IO relays.
Build the mixer library for PX4IO as well
Remove the unused complex-multirotor setup ioctl, since it's not implemented anywhere.
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.
Fix PX4IO to run C++ static ctors
Add the ability to reset a mixer group. Report the remaining buffer size from load_from_buf.
Rough in the new mixer path for PX4IO.
Fix some scaling errors in the PWM <-> mixer-internal conversions.
Factory method for a simple mixer that converts PWM/PPM values to the standard internal format.
whitespace/formatting
Assorted compile fixes.
Rename the FMU->IO output controls to reflect the fact that they are controls, not servo values.
Fix logic for handling partial buffers.
Add a receive error counter for debugging purposes.
Implement the remaining pieces of mixer upload to PX4IO.
Fix px4io signal test command to force FMU armed state.
Removed text reuse, causing crash with stack trace
Revert "Removed text reuse, causing crash with stack trace"

This reverts commit 668d1b330114fefc0ae7a6c476074f2c263c1476.
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.
Add FIONWRITE to allow applications to sniff the amount of writable space on a descriptor. Implement this for serial devices only.