tcpwrappers /etc/hosts.deny permissions bug fix

Bug fix to not modify /etc/hosts.deny permissions when removing
tcpwrappers auto-block rules. This issue was reported as Debian bug #724267
(http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=724267) and relayed via
Franck Joncourt. Closes issue #7 on github.