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

include/you.h

@@ -358,7 +358,6 @@ struct you {
     d_level utolev;     /* level monster teleported you to, or uz */
     uchar utotype;      /* bitmask of goto_level() flags for utolev */
     d_level ucamefrom;  /* level where you came from; used for tutorial */
-    boolean nofollowers; /* level change ignores monster followers/pets */
     boolean umoved;     /* changed map location (post-move) */
     int last_str_turn;  /* 0: none, 1: half turn, 2: full turn
                          * +: turn right, -: turn left */