ash: an unset dynamic variable should not be dynamic

Commit b28d4c346 (ash: [VAR] Move unsetvar functionality into setvareq)
dropped the code that caused dynamic variables to lose their special
properties when unset.  Add it back again.

function                                             old     new   delta
setvareq                                             346     360     +14
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 14/0)               Total: 14 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash.c

@@ -2364,6 +2364,10 @@ setvareq(char *s, int flags)
 		}
 
 		flags |= vp->flags & ~(VTEXTFIXED|VSTACK|VNOSAVE|VUNSET);
+#if ENABLE_ASH_RANDOM_SUPPORT
+		if (flags & VUNSET)
+			flags &= ~VDYNAMIC;
+#endif
 	} else {
 		/* variable s is not found */
 		if (flags & VNOSET)