CVE-2018-1000156/patch specifically assigned for GNU patch

Queried MITRE which informed they can either update the desciption to
match GNU patch. OTOH, DWF project has already assigned CVE-2018-1000156
for specifically GNU patch with the reason that as both have though same
root, the code has substantially diverged over time by now, thus the
seprate CVE id.

Follow that decision by marking CVE-2015-1418 NFU (specifically for
patch in FreeBSD) and add CVE-2018-1000156 entry for patch.

Updated https://bugs.debian.org/894993 accordingly.