Refactor setting roll servo canard angle out of rc_receive_imu.

Otherwise I was confused about what the current roll-control demo code
was meant to do.
Wrap enable_servo/armed in setter functions.

I want this so there's only one place to add reset code for roll-control
state.

Bonus bug-fix: initialize `armed` in rollcontrol_init. Being global, it
was already implicitly 'false', but I guess we'll be explicit about it.
Bugfix: Give each module its own "sd" file-descriptor.

I confirmed with `nm fc` that the linker or LTO was merging the three
variables into a single allocated int.
Bugfix: timestamps should use CLOCK_MONOTONIC.

If for some reason the real-time clock is slewed during flight (which
would be weird) then our packet timestamps shouldn't also change.