qemu-binfmt-conf.sh: improvements for mips

[Andreas K . Hüttel]

Re-sending v3 unchanged as requested.

The first patch has already been submitted earlier and is unchanged from v2.
The second patch extends it and resolves issue 843, "duplicate magic mips 
patterns".
Tested with various self-bootstrapped Gentoo chroots and in production on 
the Gentoo release engineering stage builder.