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/commands/files.lua

@@ -1,37 +1,11 @@
 local core = require "core"
 local command = require "core.command"
-
-local function mkdirp(path)
-  local segments = {}
-  local pos = 1
-  while true do
-    local s, e = string.find(path, "[/\\]+", pos)
-    if not s then break end
-    table.insert(segments, string.sub(str, pos, s - 1))
-    pos = e + 1
-  end
-  table.insert(segments, string.sub(str, pos))
-  if segments[#segments] == '' then
-    table.remove(segments)
-  end
-
-  for i = 1, #segments do
-    local p = table.concat(segments, PATHSEP, 1, i)
-    local stat = system.get_file_info(p)
-    if stat and stat.type == "file" then
-      return nil, "path exists", p
-    end
-    local success, err = system.mkdir(p)
-    if not success then
-      return nil, err, p
-    end
-  end
-end
+local common = require "core.common"
 
 command.add(nil, {
   ["files:create-directory"] = function()
     core.command_view:enter("New directory name", function(text)
-      local success, err, path = mkdirp(text)
+      local success, err, path = common.mkdirp(text)
       if not success then
         core.error("cannot create directory %q: %s", path, err)
       end