efibootmgr: Add option to request booting to the firmware user interface

The OsIndications UEFI variable can request the firware to stop at
its UI instead of continuing with boot. Add flags for setting and
clearing this request.

Reviewed by: manu, bcr (manpages)
Approved by: scottl (implicit)
MFC after: 1 week
Sponsored by: Ampere Computing, Inc.
Differential Revision: https://reviews.freebsd.org/D25839