xsparse cleanup, including integer overflow

xsparse cleanup, including integer overflow

* scripts/xsparse.c: Include inttypes.h, for strtoimax.
Don’t include stdint.h, since inttypes.h includes it.
Sort include directives.
Make all extern functions and vars static, except for ‘main’.
(string_to_off): Use strtoimax instead of doing overflow
checking by hand, incorrectly (it relied on undefined behavior).
(string_to_size): New arg MAXSIZE.  All callers changed.
(get_var): Return bool not int.  Fix unlikely integer overflow.
Use strncmp instead of memcmp, to avoid unlikely pointer overflow.
(read_xheader, read_map, main): Avoid unlikely integer overflow.
Check for I/O errors more consistently.
(main): Prefer bool to int, and put vars near use.