Remove BSD and USL copyright and update license block in in_prot.c
Move cr_can* prototypes from sys/systm.h to sys/proc.h
These changes are based on the following comments by rwatson:
"in_prot.c doesn't contain either BSD or USL code -- you can just reduce it
to a Robert N. M. Watson copyright with bad-style-copyright from
/usr/share/examples.
Probably the cr_can*() prototypes should be in proc.h with the other
similar checks."