curs_bkgd(3) on FreeBSD

The text 180ddc7f4b3e, as FreeBSD 2.1.7.1-RELEASE, FreeBSD 2.1.7-RELEASE, FreeBSD 2.1.5-RELEASE, FreeBSD 2.0.5-RELEASE shipped it.

ncurses window background manipulation routines

curs_bkgd(3) on FreeBSD now