Search a substring in a string
str - input string
substr - substring to search
icase - is case-insensitive search?
returns: 0..n - index in the 'str' where the substring starts
-1 - if the substring is not found
Thread-safe function.
long strstr(string str, string substr, bool icase=false);
#define stristr(str, substr) strstr((str), (substr), 1)