BUG=chrome-os-partner:2394
TEST=gmerge modemmanager, watch logs, see detected as Samsung modem, connect to AT&T network
Review URL: http://codereview.chromium.org/6614026
Patch from Jun Woo Lee <jw86.lee@samsung.com>.
Change-Id: I913628ff4a1cd16c8180e3c808644b0134e69e31
So many modems just don't implement carrier detect that it makes
the previous 'connected' logic useless, so base it off connect/disconnect
and fix up a few places that didn't check connect status before
trying to send commands. Also ensure the serial port is unlocked
for PPP to use when connected.