Remove sys-kernel/linux-btrfs-next

I'm running main-line git now. If btrfs sprouts a new hot feature
(and I hope it does), this can always be resurrected.
Add incomplete sys-kernel/linux-pf

It lacks a good configuration file (the current one is just a copy
of the main-line config file).
Move patching from exlib to exheres

Patches for one kernel seldom apply cleanly to another. It is now
the exheres's responsibility to download and apply patches.

If this ever leads to significant duplication (I tend to stick to
one main kernel), I'll think about wrapping this in a linux-patch
or similarly named exlib.
Remove all reference to removed "optimise" option
Work around kernel-install

It is crippled by design.
Routine update