Add dhclient_bash_env.rb (Bash exploit)

This module exploits a code injection in specially crafted environment
variables in Bash, specifically targeting dhclient network configuration
scripts through the HOSTNAME, DOMAINNAME, and URL DHCP options.
Add a module to exploit bash via DHCP, lands #3891

This module is just a starting point for folks to test their DHCP client implementations and we plan to significantly overhaul this once we get a bit of breathing room.