freebsd32_setcred: Copy all of the setcred fields individually

This is the more typical style used in compat syscalls. Modern
compilers are smart enough to coalesce multiple member assignments
into a bulk copy.

Reviewed by: olce, brooks
Obtained from: CheriBSD
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D53757