Add support for getting and setting BBU properties related to battery
relearning. Specifically, add subcommands to mfiutil(8) which allow the
user to set the BBU and autolearn modes when the firmware supports it,
and add a subcommand which kicks off a battery relearn.

Reviewed by: sbruno, rstone
Tested by: sbruno
Approved by: rstone (co-mentor)
MFC after: 2 weeks
Sponsored by: Sandvine Incorporated