Add a DBus interface for setting the log level.

Lifted almost entirely from similar code in NetworkManager.

BUG=chromium-os:15197
TEST='dbus-send --print-reply --system --dest=org.freedesktop.ModemManager /org/freedesktop/ModemManager org.freedesktop.ModemManager.SetLogging string:DEBUG'
Also try valid log levels 'ERR', 'WARN', 'INFO', and an invalid log level, such as 'ABCDE'.

Change-Id: I2bddcd0319f4966dd293b119f68e7cc1697949b7
Reviewed-on: http://gerrit.chromium.org/gerrit/3134
Tested-by: Nathan J. Williams <njw@chromium.org>
Reviewed-by: Eric Shienbrood <ers@chromium.org>

src/mm-log.h

@@ -47,6 +47,8 @@ void _mm_log (const char *loc,
               const char *fmt,
               ...)  __attribute__((__format__ (__printf__, 4, 5)));
 
+gboolean mm_log_set_level (const char *level, GError **error);
+
 gboolean mm_log_setup (const char *level,
                        const char *log_file,
                        gboolean show_ts,