Implement timestamps so i4b/driver/i4b_isppp.c can derive the idle

Implement timestamps so i4b/driver/i4b_isppp.c can derive the idle
time from the PPP packets sent.  This effectively merges rev 1.2 of
the old i4b_ispppsubr.c, with the exception that i eventually ended up
in debugging and fixing it so the idle time is now really
detected. ;-)  (The version in i4b simply doesn't work right since it
still accounts for incoming LCP echo packets which it is supposed to
ignore for idle time considerations...)

Obtained from:	i4b
MFC after:	1 month