Added background manipulation routines bkgd and bkgdset.

(The two actually have quite different behavior, as illustrated by my
play in ncurses-intro: bkgd changes whole existing background (or at
least attempts to; it may err), while bkgdset establishes the
background for future output (and cannot err, apparently).
Removed now pointless force_new field.

(Though of course it would need to be put back if we ever removed the
other priv field(s) from Context.)
Small doc fix.
Extend the doc.