3b2: Refactoring in preparation for Rev 3

Refactor in preparation for the addition of a Rev 3 simulator for the
3B2/1000 system.

This change also includes a full cleanup of the rat's-nest of includes
and externs that plagued the 3B2 simulator and made it difficult to
understand and maintain. Headers are now required in the following
order:

  compilation unit -> "3b2_defs.h" -> {... dependencies ...}

Finally, HELP has been added to the CPU device.

3B2/3b2_400_sys.h

@@ -0,0 +1,42 @@
+/* 3b2_400_sys.h: AT&T 3B2 Model 400 system-specific logic headers
+
+   Copyright (c) 2017, Seth J. Morabito
+
+   Permission is hereby granted, free of charge, to any person
+   obtaining a copy of this software and associated documentation
+   files (the "Software"), to deal in the Software without
+   restriction, including without limitation the rights to use, copy,
+   modify, merge, publish, distribute, sublicense, and/or sell copies
+   of the Software, and to permit persons to whom the Software is
+   furnished to do so, subject to the following conditions:
+
+   The above copyright notice and this permission notice shall be
+   included in all copies or substantial portions of the Software.
+
+   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+   MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+   BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+   ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+   CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+   SOFTWARE.
+
+   Except as contained in this notice, the name of the author shall
+   not be used in advertising or otherwise to promote the sale, use or
+   other dealings in this Software without prior written authorization
+   from the author.
+*/
+
+#ifndef _3B2_400_SYS_H_
+#define _3B2_400_SYS_H_
+
+#include "sim_defs.h"
+
+void full_reset();
+t_stat sim_load(FILE *fileref, CONST char *cptr, CONST char *fnam, int flag);
+t_stat parse_sym(CONST char *cptr, t_addr addr, UNIT *uptr, t_value *val,
+                  int32 sw);
+t_stat fprint_sym(FILE *of, t_addr addr, t_value *val, UNIT *uptr, int32 sw);
+
+#endif /* _3B2_400_SYS_H_ */