(cygheap_user::token): Delete.
(cygheap_user::impersonated): Delete.
(cygheap_user::external_token): New member.
(cygheap_user::internal_token): New member.
(cygheap_user::impersonation_state): New member.
(cygheap_user::issetuid): Modify.
(cygheap_user::token): New method.
(cygheap_user::deimpersonate): New method.
(cygheap_user::reimpersonate): New method.
(cygheap_user::has_impersonation_tokens): New method.
(cygheap_user::close_impersonation_tokens): New method.
* dtable.cc (dtable::vfork_child_dup): Use new cygheap_user methods.
* fhandler_socket.cc (fhandler_socket::dup): Ditto.
* fork.cc (fork_child): Ditto.
(fork_parent): Ditto.
* grp.cc (internal_getgroups): Ditto.
* security.cc (verify_token): Ditto.
(check_file_access): Ditto.
(cygwin_set_impersonation_token): Detect conflicts. Set
user.external_token.
* spawn.cc (spawn_guts): Use new cygheap_user methods.
* syscalls.cc (seteuid32): Rearrange to use the two tokens
in cygheap_user.
(setegid32): Use new cygheap_user methods.
* uinfo.cc: (internal_getlogin): Ditto.