geom(8): list geoms with /dev/ prefix

Allow geom(8) to list geoms with the '/dev/' prefix.

geom part show accepts the '/dev/' prefix but geom part list does not.

Modify find_geom() in sbin/geom/core/geom.c to be consistent with the behavior
of find_geom() in lib/geom/part/geom_part.c.

PR: 188213
Reported by: Ronald F. Guilmette <rfg@tristatelogic.com>
Reviewed by: imp, kevans
Approved by: kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D27556

(cherry picked from commit f200cc255f8e90b1905d94d0506a22f5dcd467c0)