Add new option to the geom(8) utility, "-p". It makes it easy to look up
the GEOM class instance from the provider name.

Reviewed by: oshogbo, 0mp
Approved by: re (kib)
MFC after: 2 weeks
Relnotes: yes
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17116