serial: rework 'connected' logic

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.

src/mm-errors.h

@@ -20,7 +20,8 @@ GType  mm_serial_error_get_type (void);
 
 enum {
     MM_MODEM_ERROR_GENERAL = 0,
-    MM_MODEM_ERROR_OPERATION_NOT_SUPPORTED = 1
+    MM_MODEM_ERROR_OPERATION_NOT_SUPPORTED = 1,
+    MM_MODEM_ERROR_CONNECTED = 2
 };
 
 #define MM_MODEM_ERROR (mm_modem_error_quark ())