Replace selwakeup() with KNOTE() in pppac(4) and pppx(4)

Replace selwakeup() with KNOTE() in pppac(4) and pppx(4)

Use the same mutex for read and write side klists. It would be overkill
to have dedicated locks.

Remove klist_invalidate() from pppacclose() because pppac(4) does not
have forced device detach. When the close routine gets called, there
should be no open file descriptors pointing the device, and consequently
the klists should be empty.

OK mvs@