Use an unsigned type for bitmask values

As noted by clang -Wshift-sign-overflow, the expression "1<<31"
overflows the signed int type, giving undefined behavior.

Use an unsigned type to make the result of the shift defined
behavior by the C99 standard.

Also change an instance of "1<<31" that was checking for whether the
most significant bit was set, as it's still undefined behavior.

Change-Id: I8cf9443aa92470181044fc3b63d491da18ff5e34
Reviewed-on: http://gerrit.openafs.org/11301
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>