Merge all MD sf_buf allocators into one MI, residing in kern/subr_sfbuf.c
The MD allocators were very common, however there were some minor
differencies. These differencies were all consolidated in the MI allocator,
under ifdefs. The defines from machine/vmparam.h turn on features required
for a particular machine. For details look in the comment in sys/sf_buf.h.

As result no MD code left in sys/*/*/vm_machdep.c. Some arches still have
machine/sf_buf.h, which is usually quite small.

Tested by: glebius (i386), tuexen (arm32), kevlo (arm32)
Reviewed by: kib
Sponsored by: Netflix
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