Integrate mkdirp function in common module (#265)

Move the function mkdirp into common to be generally available.

Use the new common.mkdirp from create_user_directory() from
core/init.lua replacing previous parent directory creation code
within the function.

The previous mkdirp function did not work on Windows where
absolute paths starts with a drive letter. The code from
create_user_directory() did not have this problem but was wrong
in the way it was creating the nested directories.

The new implementation in common.mkdirp fix both problems.

data/core/common.lua

@@ -273,4 +273,26 @@ function common.relative_path(ref_dir, dir)
 end
 
 
+function common.mkdirp(path)
+  local stat = system.get_file_info(path)
+  if stat and stat.type then
+    return false, "path exists", path
+  end
+  local subdirs = {}
+  while path and path ~= "" do
+    local success_mkdir = system.mkdir(path)
+    if success_mkdir then break end
+    local updir, basedir = path:match("(.*)[/\\](.+)$")
+    table.insert(subdirs, 1, basedir or path)
+    path = updir
+  end
+  for _, dirname in ipairs(subdirs) do
+    path = path and path .. PATHSEP .. dirname or dirname
+    if not system.mkdir(path) then
+      return false, "cannot create directory", path
+    end
+  end
+  return true
+end
+
 return common