efivar: support device paths as well as mounted paths in path_to_dp

In path_to_dp, allow passing in either the actual device path "eg
/dev/foo/bar" or the path where the device is mounted (say
/mnt/baz/bing). In the former case we'll assume the path within the
device is nothing (the relpath). In the latter, we'll take from the
mount point on down as the relpath.

Sponsored by: Netflix
Reviewed by: corvink, manu, asomers
Differential Revision: https://reviews.freebsd.org/D38616

(cherry picked from commit 57d5ca4eeba6192e91044aad86fca4429966cfac)