[perl #112990] Simplify kill implementation and docs

[perl #112990] Simplify kill implementation and docs

Clean up kill implementation and clear up the docs in perlfunc to be less
ambiguous and encompass more of its behaviour.

a)   kill -INT => $pid;

was surprisingly doing a "kill 0, $pid" instead of being the same as "kill -2,
$pid" (killing the process group for $pid with an interrupt).  Now negative
signal names will be allowed and be the same as if the name was replaced with
the signal number it represents.

b) remove all calls to killpg() as killpg is defined in terms of kill anyway.

c) Clarify the use of signal names vs numbers in perlfunc so that using names
is not so well hidden, as well as explaining the usage of negative signal
numbers as well as negative process IDs.