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When receiving a SIGCONT, don't call regenerate_screen() directly
but act as if a SIGWINCH occurred.  Also insert a dummy key, and
thus let the relevant input routine redraw the relevant subwindows.

Use KEY_F0 as the dummy key, as both Ncurses and Slang know it, and
it should normally do nothing at all.

This fixes https://savannah.gnu.org/bugs/?51124.