* pinfo.h (winpids::pid_access): New element.

* pinfo.h (winpids::pid_access): New element.
(winpids::winpids): Rejigger to set pinfo_access.
* pinfo.cc (winpids::add): Try to open shared memory region with supplied
pinfo_access first, then default to readonly.
* fhandler_termios.cc (tty_min::kill_pgrp): When getting list of pids to work
with, suggest opening with PID_MAP_RW.
* signal.cc (kill_pgrp): Ditto.
* sigproc.cc (sig_send): Perform a write check on todo prior to attempting to
increment it.  Return EACCES if we can't write to it.