std.process: add function to enforce single-instance

[expikr]

Some apps assume they only ever have one instance of themselves running on a given system.

It might make sense to provide this functionality under the std.process namespace if the interface can be cleanly mapped to platform-specific equivalents.

Windows provides CreateMutex for a global lock that can be checked against, proof of concept:

const std = @import("std");
const win = std.os.windows;

extern "kernel32" fn CreateMutexA(?*const win.SECURITY_ATTRIBUTES, win.BOOL, win.LPCSTR) callconv(win.WINAPI) ?win.HANDLE;
extern "user32" fn MessageBoxA(?win.HANDLE, ?win.LPCSTR, ?win.LPCSTR, win.UINT) callconv(win.WINAPI) i32;

fn win32_singleton(name: win.LPCSTR) error{ Unexpected, AlreadyExists }!win.HANDLE {
    const m = CreateMutexA(null, 1, name);
    const e = win.kernel32.GetLastError();
    if (m) |h| {
        if (e == .SUCCESS) return h;
        if (e == .ALREADY_EXISTS) {
            win.CloseHandle(h);
            return error.AlreadyExists;
        }
    }
    return win.unexpectedError(e);
}

pub fn main() !void {
    _ = try win32_singleton("Hello World Program");
    _ = MessageBoxA(null, "world", "hello", 0);
}