auth: Don't overflow hostName array

afsconf_cell's hostName structure is a fixed length. Don't overflow
it by writing whatever comes back from gethostbyaddr into it. Use
strlcpy to catch an overflow, and if one occurs, just use
"UNKNOWNHOST", rather than a truncated host name.

Caught by coverity (#985906)

Change-Id: Iaa927f3e4860d99166789e8dc4950a03ea2237e4
Reviewed-on: http://gerrit.openafs.org/9354
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>