Allow custom userdir directives

UserDir can be various forms of strings (see
https://httpd.apache.org/docs/2.4/howto/public_html.html) whereas
Directory must be an absolute path. This allows UserDir to be
specified separately.

The unit test showing multiple values for $path seems to be incorrect
according to https://httpd.apache.org/docs/2.4/mod/core.html#directory