Don't use #elif. Some old C compilers don't recognize it.

Don't use #elif.  Some old C compilers don't recognize it.
(hex_digits): New function.
(result): Give ctx parameter the `const' attribute.
(usage): Rewrite to make it clearer that there are three modes.
(main): Process all options before generating any output.
In particular, collect all --string options rather than operating
on them as they are encountered.

Allow --check=-.  Before it tried to open the file named `-'.

Detect bogus input when --check'ing: non-hex, non-[tb] flag.

Map to lower case all hex digits read from check file.