Preserve hard & symbolic links when modifying source file

Strip is often used to modify existing files, rather than creating new
files. If the existing file has hard links or is a symbolic link, act as
if editing the file in place and preserve the links.

Reported by: luigi
Reviewed by: imp, rpaulo
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1682