Avoid a race between MicroB startup and establishing D-Bus watch for it

As it stands, the establishment of the D-Bus match and filter for the
acquisition of com.nokia.osso_browser happens in parallel with MicroB
startup; as a result, it's possible that MicroB could acquire the name
before we're ready to look for the event, which results in us waiting
forever for MicroB to start (even though it's already started).  Avoid
this by establishing the watch before we fork a child process.

Possibly fixes a rare failure mode reported by Faheem Pervez (qwerty12).