When fork finally fails although both CreateProcess and creating the
"cygpid.N" shared memory section succeeded, we have to release that
shared memory section as well - before releasing the process handle.
Otherways we leave an orphan "cygpid.N" shared memory section, and any
subsequent cygwin process receiving the same PID fails to initialize.
* fork.cc (frok::parent): Call child.allow_remove in cleanup code.