win32: Fix process handle vs pid distinction

On win32, a process is identified by a HANDLE, but for identifying
a running process we should use GetProcessId() to return a meaningful integer.

This means we need to be able to back and forth between a pid and a process handle
(phandle). We also need to be careful to get the pid before the process handle closes
since it isn't available afterwards.

Also call the handle to intptr_t for open_osfhandle() to avoid a compiler warning.

Fixes #217

Signed-off-by: Steve Bennett <steveb@workware.net.au>