Restore handling of PMTU discovery, removed through an unifdef(1)
following the MFV of r254219 into r255332. In addition the 'FreeBSD'
macro was never defined in ipfilter 5.1.2 thus it never would have
been enabled in the first place.

This work is prompted by a general cleanup of the IP Filter code
prompted by working to resolve a PR. More to follow.

MFC after: 1 week