Minor cleanups for Lisp objects and symbols

* alloc.c (next_vector, set_next_vector):
* lisp.h (lisp_h_INTEGERP, make_number, XFASTINT, make_natnum):
(lisp_h_make_number) [USE_LSB_TAG]:
Use Lisp_Int0 instead of the mystery constant 0.
* alloc.c (mark_object): Always set and use po; that's simpler.
(CHECK_LIVE, CHECK_ALLOCATED_AND_LIVE):
Properly parenthesize definientia.
* bidi.c (bidi_initialize):
* buffer.c (init_buffer_once):
* nsfns.m (syms_of_nsfns):
* nsmenu.m (syms_of_nsmenu):
* nsselect.m (syms_of_nsselect):
Prefer DEFSYM to defining by hand.
* data.c: Fix too-long line.
* lisp.h (DECLARE_GDB_SYM): New macro.
(DEFINE_GDB_SYMBOL_BEGIN): Use it.
(DEFINE_GDB_SYMBOL_BEGIN, DEFINE_GDB_SYMBOL_END) [!MAIN_PROGRAM]:
Declare the symbol, so it's visible to everywhere lisp.h is included.
Move forward decls as far forward as they can go,
to allow future changes to use them.