MFC r314568 (by emaste):
kern_sig.c: ANSIfy and remove archaic register keyword
Sponsored by: The FreeBSD Foundation
MFC r318389 (by emaste):
Remove register keyword from sys/ and ANSIfy prototypes
A long long time ago the register keyword told the compiler to store
the corresponding variable in a CPU register, but it is not relevant
for any compiler used in the FreeBSD world today.
ANSIfy related prototypes while here.
Reviewed by: cem, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10193