This way @rmtry is now rootdir friendly
It also allows cross installation (arm rootdir on amd64 host for example)
Reviewed by: pormtgr (mat)
Differential Revision: https://reviews.freebsd.org/D23618
Note that this allows to control when it is executed (always in post-installation)
This makes @rmtry accept both absolute path and relative path (to latest prefix/cwd)
While here now that it is not used, remove the old PLIST_REINPLACE macro
Reviewed by: antoine
Differential Revision: https://reviews.freebsd.org/D713