freebsd-update: create a ZFS boot environment on install

Updated freebsd-update to allow it to create boot environments using
bectl should the system support it. The bectl utility was updated in
r352211 (490e13c1403f) to support a 'check' to determine if the system
supports boot environments. If UFS is used, the bectl check will fail
then no attempt will be made to create the boot environment.

If freebsd-update is run inside a jail, no attempt will be made to
create a boot environment.

The boot environment function will create a new environment using the
format: current FreeBSD kernel version and date/timestamp, example:

12.0-RELEASE-p10_2019-10-03_185233

This functionality can be disabled by setting 'CreateBootEnv' in
freebsd-update.conf to 'no'.

Discussed with: allanjude
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21892