setaudit: Add an update mode

By default, setaudit(8) overwrites the whole audit session state. For
the purpose of overwriting only a single field, e.g., the audit user,
this is inconvenient. Add -U to accomodate this case: when specified,
setaudit(8) will first fetch the current session state block and then
will only overwrite those fields specified on the command line.

Reviewed by: csjp
MFC after: 2 weeks
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D53672