Get linkchecker plugin folders. Default is
“$XDG_DATA_HOME/linkchecker/plugins/” if $XDG_DATA_HOME is set, else
“~/.linkchecker/plugins/” is also
supported for backwards compatibility, and is used if it exists.
Get the user configuration filename.
If the user configuration file does not exist, copy it from the initial
Returns path to user config file (which might not exist due to copy
:return configuration filename