Make pngtest --strict, --relax, --xfail options imply -m (multiple).

Moved testing of bad pngs into a separate tests/pngtest-badpngs script
Added the --xfail (expected FAIL) option to pngtest.c. It writes XFAIL
    in the output but PASS for the libpng test.

does not work (the eXIf chunk data can contain zeroes).

(Bug report from the OSS-fuzz project).

and png_set_eXIf_1(), respectively, to avoid breaking API compatibility
with libpng-1.6.31.

  in pngwrite.c, and made various other fixes to png_write_eXIf().
Eliminated png_ptr->num_exif member from pngstruct.h and added num_exif
  to arguments for png_get_eXIf() and png_set_eXIf().