* cygheap.h (cygheap_user::issetuid): New method.

* cygheap.h (cygheap_user::issetuid): New method.
* dtable.cc (dtable::vfork_child_dup): Use new method to determine if we are in
"setuid mode."
* fork.cc (fork_parent): Ditto.
* spawn.cc (spawn_guts): Ditto.
* syscalls.cc (seteuid32): Ditto.
(setegid32): Ditto.
* environ.cc (spenv::retrieve): (Suggested by Pierre Humblet) Do potential
recalculation of cygheap_user stuff when in setuid mode.  Return special value
when environment variable exists but should not be added.
(build_env): Don't add retrieved value to dstp if it is 'dont_add'.