Implement registration using unsolicited messages for generic GSM class (which
is disabled by default, HSO and Huawei plugins enable it).
Modify all GSM modem subclasses that used unsolicited messages to use the new
method.
* Add IpMethod property with known values ppp (default), static, DHCP.
* Rename DataDevice property to Device.
* Add GetIP4Config method. It should be implemented only when IpMethod==static.
* Update org.freedesktop.ModemManager.Modem.Gsm.Sms interface based on
Pablo Martí Gamboa's suggestions.
* Adjust MBM and HSO interfaces to take advantage of the generic Modem
interface.
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.
