Some changes to deal with new read-only pinfo shared memory regions.

[Christopher Faylor]

I made some changes to the winpids methods to propagate the PID_MAP_RW
flag to them as needed.  This should solve the "rxvt dies when you press
CTRL-C in the window" problem reported on the sickwin mailing list.

This is probably a temporary change until I get my pipe version of signals
checked in.  But I'm not quite ready for that yet.

I also changed sig_send to check for a writable todo before attempting to
use it, to prevent a SEGV.  Also a temporary measure...

cgf