Satisfy clang's aggressive strlcpy warnings

Passing something related to the length of the source as the
length argument to strlcpy triggers a warning, which is converted
to an error with --enable-checking (on FreeBSD 10.0).  The current
code is safe, since it is using the same expression that was used
to allocate the destination buffer, but switch to using a separate
variable to hold the length and use that variable for both allocation
and copying, to appease the compiler.

Change-Id: I580083d142fd19a4e7828c915b4846868fa8f917
Reviewed-on: http://gerrit.openafs.org/10818
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>