afsmonitor: Fix theoretical overflow of handler string

Don't do an unbounded copy into the thresh structure's handler
string, in case the caller has passed us a string which is too
long.

Instead, switch to strlcpy for all string copies.

Caught by coverity (#985761)

Change-Id: I80e3d35d7a9a4b57e8efc0cb0c7b2dc12f021063
Reviewed-on: http://gerrit.openafs.org/9443
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>