std.child_process: enable non-standard pipes

The user provides an array of struct with info for direction for the pipe.
Each struct also contains the space for the input and output end of pipe,
which simplifies handling.
Typically a low number of non-standard pipes are used, so no premature optimization
and complicating the usage.
Also, it is expected that the upper bound of used pipes per process is comptime-known,
so slicing a continuous chunk would be most efficient.

* provide user with function pointer to enable parsing of necessary file descriptor
  on Linux and file handles on Windows in the child process
  - They can not be comptime-set, because the Kernel or user expects the program to
    work in all cases.
* keep logic the same as for posix_spawn
* use fcntl and SetHandleInformation to prevent file leakage into
  subsequently created children
* test if both fcntl and HandleInformation were correctly set
* test non-standard stream with 1 pipe type

Note: Standard streams are expected to be inheritable by child processes, so those
streams are "intentionally leaked to the child".

Benchmarking with realistic workloads is needed to decide the final design.
Especially, since storing space for one additionally unused pipe end can be wasteful.

This is ziglang#11701 with conflicts fixed and will be used for the panic test runner.

Author: matu3ba

lib/std/build.zig

@@ -971,8 +971,8 @@ pub const Builder = struct {
         child.cwd = cwd;
         child.env_map = env_map;
 
-        const term = child.spawnAndWait() catch |err| {
-            log.err("Unable to spawn {s}: {s}", .{ argv[0], @errorName(err) });
+        const term = child.spawnAndWait(null) catch |err| {
+            log.err("Unable to spawn {s}: {s}\n", .{ argv[0], @errorName(err) });
             return err;
         };
 
@@ -1187,7 +1187,7 @@ pub const Builder = struct {
         child.stderr_behavior = stderr_behavior;
         child.env_map = self.env_map;
 
-        try child.spawn();
+        try child.spawn(null);
 
         const stdout = child.stdout.?.reader().readAllAlloc(self.allocator, max_output_size) catch {
             return error.ReadFailure;

lib/std/build/RunStep.zig

@@ -224,7 +224,7 @@ pub fn runCommand(
     if (print)
         printCmd(cwd, argv);
 
-    child.spawn() catch |err| {
+    child.spawn(null) catch |err| {
         std.debug.print("Unable to spawn {s}: {s}\n", .{ argv[0], @errorName(err) });
         return err;
     };