This will make it slightly easier to port mm_port_serial_at_command() to
use GTask, since we won't have to keep a pointer to the result in GTask
after _finish() has been called.

In most cases the caller needs the value anyway, so this doesn't add too
much hasle.

  ../src/plugins/cinterion/mm-plugin-cinterion.c:65:1: warning: no previous prototype for ‘is_port_already_tagged’ [-Wmissing-prototypes]
     65 | is_port_already_tagged (MMPortProbe *probe)
        | ^~~~~~~~~~~~~~~~~~~~~~

The Cinterion plugin is using the output of the SQPORT? to guess which
ports can be used for AT commands and data connection.

Yet at the same time the udev adds port type hints to the Cinterion
modem upon its discovery.

This commit changes the initialization in a way that from now on it
skips sending SQPORT? when the port type hints are already assigned. By
doing this we make sure that the udev port type hints are being used
when they are available. In case they are not the initialization relies
on the outout of SQPORT? as it did do far.

See: https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/782

GError structure may not be initialized after execution of
mm_3gpp_parse_cgdcont_read_response() and accessing it's
fields will cause a segmentation fault.

We also remove the plugin headers completely as they are not really
required anywhere.

We are going to allow including the plugin sources built within the
ModemManager daemon binary; moving the sources within the daemon
sources directory makes it easier.