[CFT/CFR] machine independent sf_bufs

  Hi!

  we've got a lot of common code in sys/*/*/vm_machdep.c wrt the
sf_buf allocation. I have gathered it into kern/subr_sfbuf.c.

o No MD code left in sys/*/*/vm_machdep.c.
o The arches that have physical map have their implementation in
  machine/sf_buf.h
o The arches that needs sf_bufs use subr_sfbuf.c, optionally having
  some stuff in machine/sf_buf.h

I can test only i386. I'd be grateful for testing:

arm
mips
mips64
sparc64
powerpc
i386 XEN

The test is a simple use of any applcation or test that uses sendfile(2).
The box shouldn't crash :) of course, and after end of a test there
should be no evidence of sf_buf leak (observed via netstat -m).

-- 
Totus tuus, Glebius.