issue #1027 - sticky 'nofollowers' flag

Reported by copperwater:  entering the tutorial sets 'u.nofollowers',
changing to the tutorial level saves a copy of 'u', post-level change
from entering the tutorial level resets u.followers, but subsequently
changing levels to return to the original level 1 restores 'u' from
the saved copy with has 'u.nofollowers==True', overriding the reset.

Move the nofollowers flag from 'u' to 'iflags'.  Invalides save and
bones files.

Fixes #1027

src/do.c

@@ -1522,7 +1522,7 @@ goto_level(
     set_ustuck((struct monst *) 0); /* clear u.ustuck and u.uswallow */
     set_uinwater(0); /* u.uinwater = 0 */
     u.uundetected = 0; /* not hidden, even if means are available */
-    if (!u.nofollowers)
+    if (!iflags.nofollowers)
         keepdogs(FALSE);
     recalc_mapseen(); /* recalculate map overview before we leave the level */
     /*