MFC r353644, r353646, r353663: bectl(8) origin auto-destroy

r353644:
libbe(3): add needed bits for be_destroy to auto-destroy some origins

New BEs can be created from either an existing snapshot or an existing BE.
If an existing BE is chosen (either implicitly via 'bectl create' or
explicitly via 'bectl create -e foo bar', for instance), then bectl will
create a snapshot of the current BE or "foo" with be_snapshot, with a name
formatted like: strftime("%F-%T") and a serial added to it.

This commit adds the needed bits for libbe or consumers to determine if a
snapshot names matches one of these auto-created snapshots (with some light
validation of the date/time/serial), and also a be_destroy flag to specify
that the origin should be automatically destroyed if possible.

A future commit to bectl will specify BE_DESTROY_AUTOORIGIN by default so we
clean up the origin in the most common case, non-user-managed snapshots.

r353646:
bectl(8): destroy: use BE_DESTROY_AUTOORIGIN if -o is not specified

-o will force the origin to be destroyed unconditionally.
BE_DESTROY_AUTOORIGIN, on the other hand, will only destroy the origin if it
matches the format used by be_snapshot. This lets us clean up the snapshots
that are clearly not user-managed (because we're creating them) while
leaving user-created snapshots in place and warning that they're still
around when the BE created goes away.

r353663:
libbe(3): Fix destroy of imported BE w/ AUTOORIGIN

Imported BE, much like the activated BE, will not have an origin that we can
fetch/examine for destruction. be_destroy should not return BE_ERR_NOORIGIN
for failure to get the origin property for BE_DESTROY_AUTOORIGIN, because
we don't really know going into it that there's even an origin to be
destroyed.

BE_DESTROY_NEEDORIGIN has been renamed to BE_DESTROY_WANTORIGIN because only
a subset of it *needs* the origin, so 'need' is too strong of verbiage.

This was caught by jenkins and the bectl tests, but kevans failed to run the
bectl tests prior to commit.