cmd: add flags argument to create syntax function

Add the flags argument to cmd_CreateSyntax() and update all callers.
The flags argument will be used to set command options, such as
CMD_HIDDEN.

Change-Id: Ia51be9635f262516cb084d236a9e0756f608bf16
Reviewed-on: http://gerrit.openafs.org/11430
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>