Move USE_BDB support from bsd.databases.mk to its own file.

Also:

• Deorbit WANT_BDB_VER, one can use USES=bdb:<ver> instead.
• USE_BDB can't happen after bsd.port.pre.mk because it is a USES.

PR: 208971
Submitted by: mat
Exp-run by: antoine
With hat: portmgr
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D5951