cmd: avoid piggy-backing flags in the help string

Remove the hack to piggy-back the hidden command option
in the help string argument.

Change-Id: Iedcb6b96e98b766e3ef2c87cd6e5d41874f2c0b7
Reviewed-on: http://gerrit.openafs.org/10982
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>