Basic support for building on NetBSD.

- Add NetBSD as a recognized platform without -ldl.
- Allow building with NetBSD libcurses instead of ncurses.
- Portability to NetBSD sh - use POSIX '=' instead of '=='.

src/ui/screen.c

@@ -39,6 +39,8 @@
 #include <ncursesw/ncurses.h>
 #elif HAVE_NCURSES_H
 #include <ncurses.h>
+#elif HAVE_CURSES_H
+#include <curses.h>
 #endif
 
 #include "config/preferences.h"