install: Assorted nitpickery.

install: Assorted nitpickery.

* Use `errc()` instead of manually setting `errno` before calling `err()`.
* Change one warning into a fatal error.
* Drop some unnecessary casts.
* `strlcat()` bounds checks were off-by-one.  This does not matter in
  practice because the subsequent code renders an overrun harmless.
* We were passing `SSIZE_MAX` to `copy_file_range()` instead of the
  requested size.  This only matters if we're asked to install a file
  which is still being written to while we are copying it.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D44810