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/hack.c

@@ -2840,7 +2840,7 @@ int x, y;
     if (!IS_DOOR(lev_p->typ))
         return FALSE;
     /* all rogue level doors are doorless but disallow diagonal access, so
-       we treat them as if their non-existant doors were actually present */
+       we treat them as if their non-existent doors were actually present */
     if (Is_rogue_level(&u.uz))
         return FALSE;
     return !(lev_p->doormask & ~(D_NODOOR | D_BROKEN));