SECURITY: make “unset HISTFILE” really work

additionally, make “HISTFILE=” the same, document the truncation and
re-reading process’ further and already-known bugs; this needs work

sh.h

@@ -169,9 +169,9 @@
 #endif
 
 #ifdef EXTERN
-__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.714 2015/03/01 15:23:05 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.715 2015/03/07 20:46:30 tg Exp $");
 #endif
-#define MKSH_VERSION "R50 2015/03/02"
+#define MKSH_VERSION "R50 2015/03/07"
 
 /* arithmetic types: C implementation */
 #if !HAVE_CAN_INTTYPES