Make if_slowtimo static
Rename if_watchdog to if_slowtimo

if_watchdog callbacks do a little more than what "watchdog" suggests.

Discussed with uebayasi@ (the idea originally from openbsd-tech).
In all_etypes prefer des3-cbc-sha1 over arcfour-hmac-md5 (#24c8bac)
Create if_slowtimo (if_watchdog) callout for each interface

This change is to obviate the need to run if_slowtimo callbacks that
may sleep inside IFNET_FOREACH. And also by this change we can turn
on MPSAFE of callouts individually.

Discussed with uebayasi@ and riastradh@.