Compare two asciiz strings
#include <str.h>
int str_diffn(const char* a,const char* b,size_t limit);
str_diffn returns negative, 0, or positive, depending on whether the string a[0], a[1], ..., a[n]=='\0' is lexicographically smaller than, equal to, or greater than the string b[0], b[1], ..., b[m-1]=='\0'.
If the strings are different, str_diffn does not read bytes past the first difference. The strings will be considered equal if the first limit characters match.