• Reduce diffs to RELENG_4 by specifying a full path to sysctl
• Adjust a little whitespace
• Make the distrib-dirs/mtree on DESTDIR conditional on user actually specifying a DESTDIR. This seemed like a safe way to get the right directories and permissions in the installed tree since 'make installworld' does the same thing, but in practice too many people have custom hacks that we should leave unmolested. Still need to find a way to deal with 'install -d' and permissions on nonexistent directories in the middle of the path, but this is at least no worse that it was before.

PR: bin/22661