Provide the tty-layer mutex when initializing the pps api. This allows
time_pps_fetch() to be used in blocking mode.

Also, don't init the pps api for system devices (consoles) that provide a
custom attach routine. The device may actually be a keyboard or other non-
tty device. If it wants to do pps processing (unlikely) it must handle
everything for itself. (In reality, only a sun keyboard uses a custom
attach routine, and it doesn't make a good pps device.)