keymaps: Use F20 for micmute keys to be friendly to X.org

The "micmute" key code is outside of X.org's allowed range [1].
gnome-settings-daemon interprets F20 as "microphone mute" these days [2], so
use this until X.org either gets fixed or obsoleted.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=692071
[2] https://mail.gnome.org/archives/commits-list/2013-January/msg05822.html