Temporary os_mkdtemp implementation. Use it instead of mkdtemp.

src/nvim/fileio.c

@@ -5254,7 +5254,7 @@ vim_tempname (
 # ifdef HAVE_MKDTEMP
         /* Leave room for filename */
         STRCAT(itmp, "vXXXXXX");
-        if (mkdtemp((char *)itmp) != NULL)
+        if (os_mkdtemp((char *)itmp) != NULL)
           vim_settempdir(itmp);
 # else
         /* Get an arbitrary number of up to 6 digits.  When it's

src/nvim/os/fs.c

@@ -3,6 +3,13 @@
 
 #include <assert.h>
 
+// TODO(hinidu): remove after implementing `os_mkdtemp` on top of libuv
+#ifdef WIN32
+# include <io.h>
+#else
+# include <stdlib.h>
+#endif
+
 #include "nvim/os/os.h"
 #include "nvim/ascii.h"
 #include "nvim/memory.h"
@@ -285,6 +292,21 @@ int os_mkdir(const char *path, int32_t mode)
   return result;
 }
 
+/// Create a unique temporary directory.
+/// TODO(hinidu): Implement on top of libuv. ref #850
+///
+/// @param[in,out] template Template of the path to the directory with XXXXXX
+///                         which would be replaced by random chars.
+/// @return Pointer to changed `template` for success, `NULL` for failure.
+char *os_mkdtemp(char *template)
+{
+#ifdef WIN32
+  return _mktemp(template) && os_mkdir(template, 0700) == 0 ? template : NULL;
+#else
+  return mkdtemp(template);
+#endif
+}
+
 /// Remove a directory.
 ///
 /// @return `0` for success, non-zero for failure.