sync bectl usage with man page

[yuripv]

Summary

Based on PR233526.

Test Plan

New usage output (also 80 chars-per-line friendly):

usage:  bectl {-h | -? | subcommand [args...]}
        bectl activate [-t] beName
        bectl create [-r] [-e {nonActiveBe | beName@snapshot}] beName
        bectl create [-r] beName@snapshot
        bectl destroy [-F] {beName | beName@snapshot}
        bectl export sourceBe
        bectl import targetBe
        bectl jail {-b | -U} [{-o key=value | -u key}]... {jailID | jailName}
              bootenv [utility [argument ...]]
        bectl list [-DHas]
        bectl mount beName [mountpoint]
        bectl rename origBeName newBeName
        bectl {ujail | unjail} {jailID | jailName} bootenv
        bectl {umount | unmount} [-f] beName

[kevans]

Please mark it for three day MFC or so and I'll pick it up on Wednesday if you don't.