bos: Don't overflow cellname buffer

Don't overflow the fixed sized cellname buffer when copying the
information in from the command line - instead, just use a
dynamically allocated buffer.

Caught by coverity (#985775)

Reviewed-on: http://gerrit.openafs.org/9549
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 45993e3ad55358c3e94105e2e3aa13df43f5fdd3)

Change-Id: Idb2b165c9b08f72cb57ca879ff6d61f9d556a631
Reviewed-on: http://gerrit.openafs.org/10860
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>