efibootmgr: Add --efidev (-u) to discover UEFI's device path to a dev or file

"efibootmgr --efidev unix-path" will return the UEFI device-path to the
file or device specified by unix-path. It's useful for debugging, but
may also be useful for scripting.

Sponsored by: Netflix
Reviewed by: corvink, manu
Differential Revision: https://reviews.freebsd.org/D38617

(cherry picked from commit 9a7915299484a767fbffc7234b8dc45c29954cca)