In an rfork'd or vfork'd situation where multiple processes are sharing

In an rfork'd or vfork'd situation where multiple processes are sharing
the same vmspace, and one process goes zombie, the vmspace's vm_exitingcnt
will be non-zero.  If another process then forks or execs the exitingcnt will
be improperly inherited by the new vmspace.  The solution is to not copy
exitingcnt when copying to a new vmspace.

Additionally, for DragonFly, I also had to fix a few cases where the upcall
list was also being improperly inherited.

Heads-up-by: Xin LI <delphij@frontfree.net>
Obtained-From: Peter Wemm <peter@wemm.org> (FreeBSD-5)