Merge from CheriBSD:

Rename sigprop-table constants to SIGPROP_ from SA_ to reduce the
impression of a namespace collision.

Submitted by: rwatson
Reviewed by: jhb, kib (slightly different versions)
Obtained from: CheriBSD (814ec5771cb1cb53deba317c561de62a91ae7684)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D7616