cmd: Add support for disabling specific abbrevs

cmd: Add support for disabling specific abbrevs

Sometimes, when adding a new command parameter, it's necessary to
prevent it from colliding with an existing abbreviation. This patch
adds a new command flag CMD_NOABBRV which can be set on a parameter
to indicate that it should not be considered when checking for
ambiguous abbreviations.

For example, if a command has the existing '-cell' option which is
popularly abbreviated to '-c', adding a '-config' option would
cause the existing abbreviation to stop working. However, if '-config'
is added with the NOABBRV flag set, '-c' will continue to work.

Change-Id: I3b6d718f9dd81c44fb1d10c904db6a4a0fd763b8
Reviewed-on: http://gerrit.openafs.org/4810
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>