Fix linking /rescue/rescue to multiple programs in usr.bin after r315113

I meant for the line that conditionally added in /usr/bin/nc support to
be +=', not =`. This restores hardlinks for all programs in usr.bin
specified before nc(1), e.g., bunzip2 and tar.

Reported by: Andrzej Tobola <ato@iem.pw.edu.pl>
Submitted by: Scott Allendorf <scott-allendorf@uiowa.edu>
Pointyhat to: ngie
MFC after: 3 days
Sponsored by: Dell EMC Isilon