Here is the cleanup patch I promised back in February.  Sorry it took a
while.

The effects should be purely cosmetic in 2.5.66.  However, the new
interface for the proper way to send thread-specific of process-global
signals from inside the kernel is needed for correct implementation of
some fixes to timer stuff that Ulrich told me about.

This cleans up some obsolete comments and macros in kernel/signal.c,
restores send_sig_info to its original behavior, and adds a global entry
point send_group_sig_info.  I checked all the uses of send_sig and
send_sig_info and changed a few to send_group_sig_info.

I think it would be cleanest if the whole mess of *_sig* entry points were
reduced to two or three, but I did the change that minimized the number of
callers I had to fix up.

There should be no discernible difference, since the 2.5.66 send_sig_info
function did group semantics for those signals by number already.  The only
exception to that is pdeath_signal, which I guess can be any signal number
but I deemed ought to be process-wide.

I did not change any of the calls using SIGKILL, though that does have
process-wide semantics.  There is no need to change it since SIGKILL always
kills the whole group, though the code path for send_sig(SIGKILL,...) calls
in multithreaded processes will be different now.