Be more systematic about _setjmp vs setjmp.

Be more systematic about _setjmp vs setjmp.

* alloc.c (test_setjmp, mark_stack):
* image.c (PNG_LONGJMP) [PNG_LIBPNG_VER < 10500]:
(PNG_JMPBUF) [! (PNG_LIBPNG_VER < 10500)]:
(png_load, my_error_exit, jpeg_load):
* process.c (send_process_trap, send_process):
Uniformly prefer _setjmp and _longjmp to setjmp and longjmp.
The underscored versions are up to 30x faster on some hosts.
Formerly, the code used setjmp+longjmp sometimes and
_setjmp+_longjmp at other times, with no particular reason to
prefer setjmp+longjmp.