Calculate str1 - str2 for upto n chars or upto the first 0 byte.
INPUTS
str1, str2 - Strings to compare
RESULT
The difference of the strings. The difference is 0, if both are
equal, < 0 if str1 < str2 and > 0 if str1 > str2. Note that
it may be greater then 1 or less than -1.
NOTES
This function is not part of a library and may thus be called
any time.