Modernize to C89, for better static checking.

Modernize to C89, for better static checking.

* Activate.c (XMenuActivate): Callback's first arg is readonly.
* AddPane.c (XMenuAddPane): Label is readonly.  Rename local
to avoid shadowing.
* AddSel.c (XMenuAddSelection): Help arg is readonly.  Rename local.
* Create.c (atoi, atof): Remove decls; include <stdlib.h>.
(MAX_INACT_PNUM, TILE_BUF_SIZE): Remove; unused.
(x_get_resource_string): Args are readonly.
(XAllocDisplayColor): colorName is readonly.
(XMenuCreate): def_env is readonly.  Remove unused locals.  Avoid
"else;".
* Destroy.c (XMenuDestroy): Return void.
* Error.c (XMenuError): Remove const pointer.
* EvHand.c (XMenuEventHandler): Return void.
* FindPane.c, FindSel.c: Include <string.h>.
* InsPane.c (XMenuInsertPane): Rename local to avoid shadowing.
* InsSel.c (XMenuInsertSelection): Likewise.
* Internal.c (toggle_color, BUFFER_SIZE): Remove; unused.
(_XMErrorList): Now const.
(_XMWinQueInit, _XMRecomputeGlobals, _XMTransToOrigin, _XMRefreshPane):
(_XMRefreshSelection): Return void.
(_XMWinQueFlush, _XMRefreshSelection): Rename locals to avoid
shadowing.
(_XMWinQueFlush): Use stack, not heap.  Don't use uninitialized var.
* SetAEQ.c (XMenuSetAEQ): Now returns void.
* SetFrz.c (XMenuSetFreeze): Likewise.
* X10.h (XAssoc): Use void * for generic pointer.
* XDelAssoc.c: Include XMenuInt.h rather than duplicating part of it.
* XDestAssoc.c, XMakeAssoc.c: Likewise.
* XDestAssoc.c (XDestroyAssocTable): Return void.
* XMakeAssoc.c (XMakeAssoc): Use void * for generic pointer.
* XMenu.h, XMenuInt.h: Adjust to signature changes.  Use const
for pointers to readonly storage.
* insque.c: Include XMenuInt.h, to check our own signature.
(emacs_insque, emacs_remque): Use void * for generic pointers.