Document the locking mechanism of the global variables in ARP code.

Document the locking mechanism of the global variables in ARP code.
The global list of ARP llinfo is protected by net lock.  This is
not sufficent when we switch to shared netlock.  Add a mutex for
insertion and removal when net lock is not exclusive.  This is
needed if we want run IP output on multiple CPU.
Put an assertion for shared net lock into arp_rtrequest.
input mvs@; OK sashan@