Add an unprivileged mode where calls to install are passed appropriate
flags. For ease of integration, use the same flags as install:

-U unprivileged mode
-D <destdir> Specify DESTDIR (overrides the environment)
-M <metalog> Full path to METALOG file

Reviewed by: kevans
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24932