std.ChildProcess: improvements to collectOutputPosix

* read directly into the ArrayList buffers.
 * respect max_output_bytes

 * std.ArrayList:
   - make `allocatedSlice` public.
   - add `unusedCapacitySlice`.

I removed the Windows implementation of this stuff; I am doing a partial
merge of LemonBoy's patch with the understanding that a later patch can
add the Windows implementation after it is vetted.

lib/std/array_list.zig

@@ -337,11 +337,21 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
             return self.pop();
         }
 
-        // For a nicer API, `items.len` is the length, not the capacity.
-        // This requires "unsafe" slicing.
-        fn allocatedSlice(self: Self) Slice {
+        /// Returns a slice of all the items plus the extra capacity, whose memory
+        /// contents are undefined.
+        pub fn allocatedSlice(self: Self) Slice {
+            // For a nicer API, `items.len` is the length, not the capacity.
+            // This requires "unsafe" slicing.
             return self.items.ptr[0..self.capacity];
         }
+
+        /// Returns a slice of only the extra capacity after items.
+        /// This can be useful for writing directly into an `ArrayList`.
+        /// Note that such an operation must be followed up with a direct
+        /// modification of `self.items.len`.
+        pub fn unusedCapacitySlice(self: Self) Slice {
+            return self.allocatedSlice()[self.items.len..];
+        }
     };
 }