Update zlib to 1.2.11.

libs/zlib-1.2.11/contrib/iostream3/TODO

@@ -0,0 +1,17 @@
+Possible upgrades to gzfilebuf:
+
+- The ability to do putback (e.g. putbackfail)
+
+- The ability to seek (zlib supports this, but could be slow/tricky)
+
+- Simultaneous read/write access (does it make sense?)
+
+- Support for ios_base::ate open mode
+
+- Locale support?
+
+- Check public interface to see which calls give problems
+  (due to dependence on library internals)
+
+- Override operator<<(ostream&, gzfilebuf*) to allow direct copying
+  of stream buffer to stream ( i.e. os << is.rdbuf(); )