Use relative URLs when URL_RELPREFIX is defined and URLS_ABSOLUTE
is not defined.

• URL_RELPREFIX must always point the top page's URL (http://www.FreeBSD.org) in relative form, defined in Makefile.

• &url.base; can be used in doc/<langcode>/*. It will be replaced with the real URL which points the top page.

For translators: Please make sure to define URL_RELPREFIX to
point the top page. The localized docs have different directory
hierarchy in its URL from the English version.

Tested by: den