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/init.lua

@@ -276,25 +276,12 @@ end
 -- create a directory using mkdir but may need to create the parent
 -- directories as well.
 local function create_user_directory()
-  local dirname_create = USERDIR
-  local basedir
-  local subdirs = {}
-  while dirname_create and dirname_create ~= "" do
-    local success_mkdir = system.mkdir(dirname_create)
-    if success_mkdir then break end
-    dirname_create, basedir = dirname_create:match("(.*)[/\\](.+)$")
-    if basedir then
-      subdirs[#subdirs + 1] = basedir
-    end
-  end
-  for _, dirname in ipairs(subdirs) do
-    dirname_create = dirname_create .. '/' .. dirname
-    if not system.mkdir(dirname_create) then
-      error("cannot create directory: \"" .. dirname_create .. "\"")
-    end
+  local success, err = common.mkdirp(USERDIR)
+  if not success then
+    error("cannot create directory \"" .. USERDIR .. "\": " .. err)
   end
   for _, modname in ipairs {'plugins', 'colors', 'fonts'} do
-    local subdirname = dirname_create .. '/' .. modname
+    local subdirname = USERDIR .. PATHSEP .. modname
     if not system.mkdir(subdirname) then
       error("cannot create directory: \"" .. subdirname .. "\"")
     end