Rework ppp network functionality and introduce the pppd-angel.
Enhance backtrace with called binary.

So it is easier to find out which command has been
executed.
modem: Fix getting provider information.

If there were spaces in the output string, the string has not
been correctly parsed.
Handle hotplugging of serial devices.
pppd-angel: Use a default holdoff time of 10 seconds.
pppd-angel: Enhance logging.
modem: Save IMSI (optionally).