Implement modem control in u3g. Tested on Option GTM382W, Huawei E220, and Sierra Wireless MC8790V. Also implement the .ucom_poll method.
Implement modem control in u3g. Tested on Option GTM382W, Huawei E220,
and Sierra Wireless MC8790V. Also implement the .ucom_poll method.

Note: This makes it possible to use lqr/echo in ppp.conf. And it
resolves ppp hanging during the PPp> phase.

Reviewed by:	hps
MFC after:	1 week

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