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.c

@@ -54,6 +54,7 @@ mm_modem_error_get_type (void)
         static const GEnumValue values[] = {
             ENUM_ENTRY (MM_MODEM_ERROR_GENERAL,                 "General"),
             ENUM_ENTRY (MM_MODEM_ERROR_OPERATION_NOT_SUPPORTED, "OperationNotSupported"),
+            ENUM_ENTRY (MM_MODEM_ERROR_CONNECTED,               "Connected"),
             { 0, 0, 0 }
         };