Fix fs bypassthreshold to accept a size of -1 to disable

Fix fs bypassthreshold to accept a size of -1 to disable

The fs bypassthreshold command assumes a value of -1 means the feature
is disabled, but the CLI refused to accept this argument, since it is
not strictly a digit (according ti isdigit()).  This patch accepts the
string -1, and makes it possible to both enable AND disable this
feature.

Change-Id: I87720b2dcfc4e9ee9f322c4841836b74440ac442
Reviewed-on: http://gerrit.openafs.org/3009
Reviewed-by: Matt Benjamin <matt@linuxbox.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>