Revert "Remove konqueror USE flag mask"

This reverts commit 68c5db7203f790f986570eaee2edd9eebd09feb3.
www-client/firefox: nss, mesa and libdrm are now stable
Keyword sys-libs/libcxx{,rt}-0.0_p20140322

For apps that need libc++.
app-emulation/virtualbox: 4.3.16 -> 4.3.20
Merge branch 'stage' into prime