Corrected --disable-unlinkd. It didn't correctly parse the arguments

Corrected --disable-unlinkd. It didn't correctly parse the arguments
and always enabled unlinkd..

Excended the makefile trickery used for --disable-unlinkd to also
apply to dnsserver and pinger. Now no helpers are built or installed
unless requested.

Note: The makefile trickery will probably fail on old UNIX:es if no
helpers are to be installed. Need to find a way to test if a variable
contains anything else besides whitespace.. Workaround: Don't use
--disable-unlinkd or use --disable-internal-dns.