This patch adds a 'mtu' value to the Ip4Config and Ip6Config property of
a Bearer object, which indicates the value of the maximum transmission
unit for the established connection when such information is available
(e.g. via QMI_WDS_GET_RUNTIME_SETTINGS on a QMI modem or
MBIM_CID_IP_CONFIGURATION on a MBIM modem).
It's pointless to have libmm-common around, just merge it into libmm-glib and
make ModemManager depend on libmm-glib directly. At the end, the non-common
stuff in libmm-glib is really minimal.
Renamed `MMCommonBearerIpConfig' to `MMBearerIpConfig', and removed the
`MMBearerIpConfig' provided in libmm-glib. We'll just use the original one
from libmm-common always.
