Fix path lookup stuff

We actually do need to pass non-symlinked stuff, just can't use readlink for
it. Use realpath, which is the portable alternative, instead.

tests/run/simple_test

@@ -10,7 +10,7 @@ conffile=${tmpdir}/nbd.conf
 pidfile=${tmpdir}/nbd.pid
 tmpnam=${tmpdir}/nbd.dd
 mydir=$(dirname $0)
-certdir="${mydir}/certs"
+certdir=$(realpath ${mydir}/certs)
 cleanup="$2"
 PID=""