Remove mention of some old WANT_* and USE_* variables from the Porter's Handbook

WANT_PHP_WEB
USE_FIREFOX*
USE_SEAMONKEY*
USE_THUNDERBIRD*
WANT_GNOME
HAVE_GNOME
USE_BDB
USE_PGSQL
USE_SQLITE

are all not supported anymore or have been replaced by USES=x.

Rewrite "Using Databases", add examples, and point to the various
USES macros for dealing with database dependencies.

Approved by: mat
Differential Revision: https://reviews.freebsd.org/D15704