Bug 18821: Disable libmdns for Android and Desktop

There should be no need to remove the OS X support introduced in
https://bugzilla.mozilla.org/show_bug.cgi?id=1225726 as enabling this
is governed by a preference (which is actually set to `false`). However,
we remove it at build time as well (defense in depth).

This is basically a backout of the relevant passages of
https://hg.mozilla.org/mozilla-central/rev/6bfb430de85d,
https://hg.mozilla.org/mozilla-central/rev/609b337bf7ab and
https://hg.mozilla.org/mozilla-central/rev/8e092ec5fbbd.

Fixed bug 21861 (Disable additional mDNS code to avoid proxy bypasses)
as well.