bectl(8): implement sorting for 'bectl list' output

Allow 'bectl list' to sort output by a given property name. The property
name is passed in using a command-line flag, '-c' for ascending order and
'-C' for descending order. The properties allowed to sort by are:

• name (the default output, even if '-c' or '-C' are not used)
• creation
• origin
• used
• usedds
• usedsnap
• usedrefreserv

The default output for 'bectl list' is now ascending alphabetical order of
BE name.

To sort by creation time from earliest to latest, the command would be
'bectl list -c creation'

Submitted by: Rob Fairbanks <rob.fx907 gmail com>
Reviewed by: ler
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20818