main: the base directory for `--strip-paths` should be root_fd, not cwd

main: the base directory for `--strip-paths` should be root_fd, not cwd

The cwd is where core dumps and various profiling and verification
libraries want to write their data, whereas root_fd is the root of the
target filesystem.  These are often intentionally different.  When
they are different, `--strip-paths` sets the wrong prefix to strip
from paths.

Once the root fd has been established, we can set the path prefix to
the string prefix that we'll get from future calls to `name_fd`.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>