Add support of vulnxml parsing.

Add preliminary support for vulnxml parsing.

Use portaudit variant by default and turn on vulnxml only by -x
flag passed to pkg-audit.

Add VULNXML_SITE setting to setup portaudit mirror, default is:
http://www.vuxml.org/freebsd/vuln.xml.bz2
Document -x flag.

Add documentation for -x flag to pkg-audit(1) and to self
description of the command.