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

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)
MFC after:      1 week
Differential Revision:  https://reviews.freebsd.org/D27556