If the modem becomes invalid (it crashes and resets, for example)
the callback-info's modem_destroyed_cb() function will handle
cleanup. Buf if the callback-info's callback does more work than
just returning the result (like simple_state_machine) it could
double-schedule the callback. Don't let that happen.
We need better modem-removal handling, but this fixes a crash for
now.
Rework the MMCallbackInfo callback invocation.
Always use g_error_literal() where it makes sense.
Replace sleep() calls, with timeouts to not block the whole MM.
Instead of vague "send something, wait something" the responses are now
analyzed by (overridable) parsers. Makes all the modem implementations much
easier since each caller knows without any code whether the call succeeded
or failed.
Another thing that makes modem code simpler (and the whole thing more robust),
is the queueing of sent commands. Each queued command has a command and a
callback which is quaranteed to get called, even if sending failed.
Define and implement error reporting.
