spelling and typo fixes

Submitted for 3.7.0; all but one also apply to 3.6.3.

I rewrote the curses terminal-too-small message instead of just
fixing the spelling of "minumum".

src/bones.c

@@ -598,7 +598,7 @@ getbones()
 
     if (validate(fd, bones) != 0) {
         if (!wizard)
-            pline("Discarding unuseable bones; no need to panic...");
+            pline("Discarding unusable bones; no need to panic...");
         ok = FALSE;
     } else {
         ok = TRUE;