Mark geom classes as deprecated.

geom_bsd, geom_mbr and geom_sunlabel have been obsolete since Marcel
Moolenaar's geom_part was in FreeBSD 7. They haven't been in GENERIC
since FreeBSD 8. Add warning when used.

geom_vol_ffs has been obsolete since ufs support to geom_label was
committed in FreeBSD 5. It hasn't been in GENERIC since FreeBSD 5.
Add warning when used.

geom_fox has been obsolete since gmultipath was committed in FreeBSD 7.
(no warning added, since this is a very obscure class).

These will all be removed in FreeBSD 12.

MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D11935

Note: Classes will be removed after MFC