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/flag.h

@@ -197,6 +197,7 @@ struct instance_flags {
     boolean invis_goldsym; /* gold symbol is ' '? */
     boolean in_lua;        /* executing a lua script */
     boolean lua_testing;   /* doing lua tests */
+    boolean nofollowers;   /* level change ignores pets (for tutorial) */
     boolean partly_eaten_hack; /* extra flag for xname() used when it's called
                                 * indirectly so we can't use xname_flags() */
     boolean remember_getpos; /* save getpos() positioning in do-again queue */