f-by-one errors.
  Using this function instead avoids such errors.

- some code currently tests unsigned signal numbers for <0 which is
  pointless and generates warnings when building with gcc -W.  Using this
  function instead avoids such warnings.

I considered various places to add this function but eventually settled on
include/linux/signal.h as the most logical place for it.  If there's some
reason this is a bad choice then please let me know (hints as to a better
location are then welcome of course).

Signed-off-by: Jesper Juhl <juhl-lkml@dif.dk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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