fork: remove cygpid.N sharedmem on fork failure

fork: remove cygpid.N sharedmem on fork failure

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.