Previously, a few operations (like disable) could trigger a modem
flash in parallel with another flash. That's wrong, don't allow
that. At the same time, add in finer-grained error checking on
serial port speed operations, and fix a GSM generic bug that would
send the POWER_UP string on disable.
So many modems just don't implement carrier detect that it makes
the previous 'connected' logic useless, so base it off connect/disconnect
and fix up a few places that didn't check connect status before
trying to send commands. Also ensure the serial port is unlocked
for PPP to use when connected.
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.
