Improve transtable.{xml,xsl}, including:

• The structure of transtable.xml is revised. <word> should be bracketed with <group>.
• A sorting order of the FreeBSD mirror sites rendered using mirrors.xml has been determined by sort(1).
• A template "transtable-lookup" has been added for localization on word-by-word basis.
• Replace English month names in news.xml, press.xml, and advisories.xml with numbers that correspond to the names. The number->name translation is performed on the fly.
• Since information in mirrors.xml is used in www/ tree, it depends on doc/ tree now. When WITHOUT_DOC is defined it can be built without doc/ tree, but some information becomes unavailable. For example, a list of the mirror sites generated in index.html becomes a dummy one, and calling "transtable-lookup" with a word returns the word itself.

Neither www/ nor doc/ build should be broken due to this commit, but
until the necessary changes are applied in the localized directories,
the transtable does not work; it simply generates non-localized contents
even if transtable.xsl is used.