Fix bug in return value of strcmp/strncmp

[calc84maniac]

Zilog's implementations of strcmp and strncmp oddly sign extend the difference of the two characters. This notably has the following problems:

• The C standard specifies the characters should be compared as unsigned char. Zilog's behavior is fine if all characters are in the ASCII range and thus differ by no more than 127, but larger differences may be returned with the wrong sign.
• This behavior causes strings not to have a total ordering - for example, 0x20 < 0x70 and 0x70 < 0xC0 but 0x20 > 0xC0.
• This was fixed by TI in their OS implementation, so this causes behavior different than in CEdev programs.

I was able to fix this (which ends up with better performance in the difference calculation so it's a win-win), and while I was at it, I also optimized the strncmp loop a bit. I also improved a couple of the pseudo op replacements from earlier Zilog routine edits.

I should also note that on the CEdev side, I fixed the strncasecmp errors which had been reported earlier, and also added an implementation for strcasecmp, so that should be usable in AgDev.

[astralaster]

Did not test it, but looks good.