[os2/process]: Implement process_exec, and pipe_has_data

[flysand7]

This PR implements a procedure to allow non-blocking reading of the stdout and stderr after starting the process using os2. This function should also be applicable to sockets, but for now there's no demand for this.

There's also a fix of errors on Windows, where win32.ERROR_BROKEN_PIPE wasn't properly converting the error into General_Error.Broken_Pipe, unlike linux and posix platforms.

The documentation for pipe() procedure has been added.

The documentation of os2/process functions has been unindented.

The following snippet demonstrates the usage of the procedure:

package main

import "core:os/os2"
import "core:fmt"

main :: proc() {
	cwd, cwd_err := os2.get_working_directory(context.allocator)
	if cwd_err != nil {
		fmt.printfln("Error getting CWD: %v", cwd_err)
		os2.exit(1)
	}
	state, stdout, stderr, err := os2.process_exec(os2.Process_Desc {
		command = []string {"odin", "check", "core/os/os2", "-vet", "-strict-style", "-vet-tabs"},
		env = os2.environ(context.allocator),
		working_dir = cwd,
	}, context.allocator)
	if err != nil {
		fmt.printfln("Error running process: %v", err)
		os2.exit(1)
	}
	fmt.printfln("----------------- STDOUT --------------------")
	fmt.printfln("%s", transmute(string) stdout)
	fmt.printfln("----------------- STDERR --------------------")
	fmt.printfln("%s", transmute(string) stderr)
	fmt.printfln("----------------- STATUS --------------------")
	fmt.printfln("%v", state)
}