Partially merge iostreams.

* New constructors/`open` for file descriptors.
  Fixes #3517.
* Leaving out changes which depend on filesystem v3.


[SVN r63679]

libs/iostreams

@@ -1 +1 @@
-Subproject commit b08ae2bf1f9a4ff1079f766405d56bb1b64ed71c
+Subproject commit 7914b53342e4c2fd6e5ffc8def25e1e12a18f1cc