Add zlib gzip functions

These will now be included in the cosmopolitan.a releases. It took a bit
of time because, these functions depend on heavyweight parts of the libc
that wouldn't be appropriate for the core zlib library to depend upon.

Fixes #345

third_party/zlib/gz/gzclose.c

@@ -0,0 +1,27 @@
+#include "third_party/zlib/gz/gzguts.inc"
+// clang-format off
+
+/* gzclose.c -- zlib gzclose() function
+ * Copyright (C) 2004, 2010 Mark Adler
+ * For conditions of distribution and use, see copyright notice in zlib.h
+ */
+
+asm(".ident\t\"\\n\\n\
+zlib (zlib License)\\n\
+Copyright 1995-2017 Jean-loup Gailly and Mark Adler\"");
+asm(".include \"libc/disclaimer.inc\"");
+
+/* gzclose() is in a separate file so that it is linked in only if it is used.
+   That way the other gzclose functions can be used instead to avoid linking in
+   unneeded compression or decompression routines. */
+int gzclose(file)
+    gzFile file;
+{
+    gz_statep state;
+
+    if (file == NULL)
+        return Z_STREAM_ERROR;
+    state = (gz_statep)file;
+
+    return state->mode == GZ_READ ? gzclose_r(file) : gzclose_w(file);
+}