To use bectl in an example, when one creates a new boot environment with
either `bectl create <be>` or `bectl create -e <otherbe> <be>`, libbe will
take a snapshot of the original boot environment to clone. Previously, this
used %F-%T date format as the snapshot name, but this has some limitations-
attempting to create multiple boot environments in quick succession may
collide if done within the same second.
Tack a serial onto it to reduce the chances of a collision... we could still
collide if multiple processes/threads are creating boot environments at the
same time, but this is likely not a big concern as this has only been
reported as occurring in freebsd-ci setup.
MFC after: 3 days
90cf61e8a5