Log Message:
Rework the Berkeley DB detection in buildlink3:

  * Add a db1.builtin.mk file that detects whether DB-1.85 functionality
    exists in the base system, and remove the distinction between
    "native" and the other Berkeley DB packages -- we now refer to
    db[1234].  This paves the way for any future databases/db1 package.

  * USE_DB185 shouldn't need to be set by any packages -- its correct
    value is now automatically determined by bdb.buildlink3.mk depending
    on whether we explicitly request db1 or not.  By default, if you
    include bdb.buildlink3.mk, you want DB-1.85 functionality and
    USE_DB185 defaults to "yes", but if you explicitly remove db1 from
    the list of acceptable DBs, then USE_DB185 defaults to "no".

  * Set BDB_LIBS to the library options needed to link against the DB
    library when bdb.buildlink3.mk is included.

  * We only add the DB library to the linker command automatically if
    we want DB-1.85 functionality; otherwise assume that the package
    configure process can figure out how to probe for the correct
    headers and libraries.

Edit package Makefiles to nuke redundant settings of USE_DB185.