Implement DNF flag to set/clear Do Not Fragment bit

[trmckee3]

The ability to set/clear the DNF bit is an important troubleshooting tool when looking for PMTUD issues. At this point I have to refer my techs to use pingplotter which does support DNF. This should be a relatively simple change as it involves only a flag and a single bit in the IP header. I'm not familiar enough with github projects to do this.