BUGFIX: readlink requires chdir for relative links

Fixes #2650 (although there is more work to be done).  The issue is that
`path.readlink.exist?` returned false for **all** relative links, because `readlink`
reads relative to the current working directory.  This caused relative
symlinks such as `/usr/bin/tar` to be deleted, because package `mosh` identified
`/usr/bin` as one of its container directories.