altos: Handle internal and external telem monitoring requests

altos: Handle internal and external telem monitoring requests

Record separate internal vs external monitoring state, allowing
both to happen at the same time, and when either is turned off, the
other keeps working.

This also adds disable/enable so that other radio users can
temporarily take over the radio; monitoring will resume when the other
radio user is finished.

Signed-off-by: Keith Packard <keithp@keithp.com>