ModemManager port grabbing rework

[Dan Williams <dcbw redhat com>]

Hi,

The core problem was that some devices need to specify a PPP port that's
*not* the primary port.  In these cases the primary port always stays
open for command and status, while some other port is used for PPP.
This is different than the current model where the primary port is used
for command and status until PPP is started, whereupon command and
status switches to a secondary port.  The internal MM model was too
inflexible here.

In the 'ports' branch, the core change here is to make MM_PORT_TYPE_xxx
only about types like AT, QCDM, etc.  A new flags enum is created for AT
ports specifically (since these are currently the only ones we care
about for primary/secondary) called MM_AT_PORT_FLAG_xxx which is what
plugins now use to give hints to the core about
primary/secondary/data/etc.

'ports' now uses the correct port layout on the Huawei E220, and even
makes newer Sierra devices more functional by using the limited APP
ports for PPP instead of command and status when connected.

Any review appreciated; it's a pretty big patch and it changes how
plugins tag ports and interact with the core.  If it looks good we'll
merge to git master for 0.6, and maybe merge it later for 0.5.x.

Dan