BACKENDS: fix FTBFS on GNU Hurd by replacing MAXHOSTNAMELEN with NI_MAXHOST

Bug-ScummVM: https://sourceforge.net/tracker/?func=detail&atid=418820&aid=3614268&group_id=37116

See more in similar http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=387665
Merge pull request #339 from onlyjob/hurd

BACKENDS: fix FTBFS on GNU Hurd by replacing MAXHOSTNAMELEN with NI_MAXHOST