Fix scanf buffer overflows

Fix potential buffer overflows caused by misuse of the scanf function
in the fileserver and ptserver.

Also fix similar issues in the client side fs command and libadmin
library.

Change-Id: Ia6a46981c50537da1673507c2bc777f96e43f95a
(This change was applied to the 1.6 branch as a security fix for 1.6.2 as
commit d1855f8e04; this commit brings the fix into master.)
Reviewed-on: http://gerrit.openafs.org/9962
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>