kill: split out from skill/snice

kill: split out from skill/snice

The first part of fixing skill/snice to use the library instead
of directly readdir()ing /proc which is what it does now.

Remove the kill code from the skill/snice code and put common
elements into lib/signals.c Not 100% sure that is the right
destination instead of a new lib file, but ok for now.

kill shares some parsing logic with skill/snice but mainly
around signal specifications. The "do it" code is very different.

Signed-off-by: Craig Small <csmall@enc.com.au>