Add refinements on mirrors.xml:

• add doc.common.mk, which defines variables, targets, and dependencies commonly used in www/ and doc/.
• move www/<lang>/includes.xsl to the language independent directory and split into several files.
• add transtable*.xsl and transtable.xml to support localized mirror names.
• make doc/{en_US.ISO8859-1,ja_JP.eucJP}/books/handbook/ use mirrors.xml (mirrors and eresources).
• make www/{en,ja}/index.xsl use mirrors.xml.

For details, please see doc/share/sgml/README.mirrors for the moment.

Reviewed by: simon and Alex Dupre <sysadmin@alexdupre.com>