curs_clear(3) on FreeBSD

The text 5db20a8178bb, as FreeBSD 2.1.7.1-RELEASE, FreeBSD 2.1.7-RELEASE, FreeBSD 2.1.5-RELEASE, FreeBSD 2.0.5-RELEASE shipped it.

clear all or part of a ncurses window

curs_clear(3) on FreeBSD now