printf: diagnose empty args correctly

printf: diagnose empty args correctly

Also, port better to macOS.
* src/printf.c (verify_numeric): Don’t assume that when s == end
then errno is zero; it is EINVAL on macOS, and POSIX allows this.
(print_direc): Treat missing arg as zero for numeric conversions,
and as an empty string for the others.
(print_formatted): Use null pointer, not an empty string,
to represent missing arg.
* tests/printf/printf.sh: Test empty and space widths and precisions.