install: handle -m +X more accurately

As described by chmod(1), +X in the mode may be used to optionally set
the +x bit if the file is a directory if any of the execute/search bits
are set in the original mode. The latter is not applicable because we
assume -m is a fresh mask, but a functional +X could be useful in the
former case if we're passing along a common INSTALL_MODE that's designed
to install either 0644 or 0755 depending simply on whether it's a
directory or not.

Reviewed by: des
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D42273