CHERI: declare mem{cpy,move}_data

Declare kernel-only, provenance-discarding memcpy_data, and memmove_data
APIs intended to copy raw data which does not contain pointers (e.g.,
buffers on their way to or from network or storage devices). On CHERI
architectures, they will explicitly remove tags from capabilities,
removing any provenance. This reduces the risk of accidental spread of
pointers on CHERI systems.

Document that bcopy preserves pointer provenance.

Reviewed by: ziaee, kib, adrian, markj
Effort: CHERI upstreaming
Sponsored by: DARPA, AFRL, Innovate UK
Differential Revision: https://reviews.freebsd.org/D57662