bugfix: create a valid hostname from buildname

mail/postfix mysteriously failed to build at do-install phase without
any error message. it was caused by my build name, which happens to be
10.0-foo at bar. the hostname becomes jail's hostname, which is not a
valid DNS name. because of it, postfix bails out.

http://www.marcuscom.com/pipermail/tinderbox-list/2014-April/003387.html