Name space cleanup in generated parser.

Name space cleanup in generated parser.
* doc/bison.texinfo (Bison Parser): Discuss system headers
and their effect on the user name space.
* src/bison.simple:
(YYSTACK_ALLOC, YYSTACK_FREE, union yyalloc, YYSTACK_GAP_MAX,
YYSTACK_BYTES, YYSTACK_RELOCATE): Do not define unless necessary,
i.e. unless ! defined (yyoverflow) || defined (YYERROR_VERBOSE).
(YYSIZE_T): New macro.  Use it instead of size_t, to avoid infringing
on user names when possible.
(YYSTACK_USE_ALLOCA): Do not define; just use any existing defn.
Simplify test for whather <alloca.h> exists.
(<stdlib.h>): Include if we will use malloc, and if standard C or C++.
(<stdio.h>): Include if YYDEBUG.
(yymemcpy): Renamed from __yy_memcpy.  Do not define unless
! defined (yyoverflow) && ! defined (yymemcpy).
(yymemcpy, yyparse): Rename local variables as needed so that
they all begin with 'yy'.
(yystrlen, yystpcpy): New functions.
(YY_DECL_NON_LSP_VARIABLES): Renamed from _YY_DECL_VARIABLES.
All uses changed.
(yyparse): size_t -> YYSIZE_T.  Use yystrlen and yystpcpy
instead of relying on string.h functions.  Use YYSTACK_ALLOC
and YYSTACK_FREE instead of malloc and free.