core::run and rust_run_program.cpp do unsafe and leaky things on windows

[Dretch]

When starting a child process on Windows then rust_run_program.cpp casts a process HANDLE to int and then core::run pretends that it is really a process id (a pid_t).

This is bad because (on Windows):

• The HANDLE never gets closed (it leaks).
• It means calling core::os::waitpid or core::run::waitpid with a pid_t that wasn't retrieved from rust_run_program will result in undefined behaviour.
• it means that calling core::run::Program.get_id returns a value that isn't really the process id.

I will send a pull request to address these issues, unless there are objections.

[graydon]

Please do! Sorry about the current behavior

[mzabaluev]

The fix bisects as the cause of #6096.