MFC r329273, r329275, r329277, r329284, r329344

r329273:
geli: append "/eli" to the underlying provider's physical path

If the underlying provider's physical path is null, then the geli device's
physical path will be, too. Otherwise, it will append "/eli". This will make
geli work better with zfsd(8).

PR: 224962
Differential Revision: https://reviews.freebsd.org/D13979

r329275:
gpart: append partition name to the underlying provider's physical path

If the underlying provider's physical path is null, then the gpart device's
physical path will be, too. Otherwise, it will append the partition name,
such as "/p1" or "/s1/a". This will make gpart work better with zfsd(8).

PR: 224965
Differential Revision: https://reviews.freebsd.org/D14010

r329277:
Add mtree entry for 329275

X-MFC-With: 329275
Sponsored by: Spectra Logic Corp

r329284:
zfsd: Allow zfsd to work on any type of GEOM provider

cddl/usr.sbin/zfsd/zfsd_event.cc
Remove the check for da and ada devices. This way zfsd can work on md,
geli, glabel, gstripe, etc devices. geli in particular is useful
combined with ZFS. gnop is also useful for simulating drive pulls in
the ZFSD test suite.

Also, eliminate the DevfsEvent class entirely. Move its
responsibilities into GeomEvent. We can get everything we need to know
just from listening to GEOM events.

lib/libdevdctl/event.cc
Fix GeomEvent::DevName for CREATE events. Oddly, the relevant field is
named "cdev" for CREATE events but "devname" for disk events.

Relnotes: Yes (probably worth mentioning the geli part)
Sponsored by: Spectra Logic Corp

r329344:
Optimize zfsd for the happy case

If there are no damaged pools, then ignore all GEOM events. We only use
them to fix damaged pools. However, still pay attention to ZFS events.

X-MFC-With: 329284
Sponsored by: Spectra Logic Corp