Unify unsigned integer usage in the API

Replace all occurences of unsigned int and unsigned long with size_t.

This is a backwards incompatible change, as the signature of many API
functions changes.

doc/github_commits.c

@@ -96,7 +96,7 @@ static char *request(const char *url)
 
 int main(int argc, char *argv[])
 {
-    unsigned int i;
+    size_t i;
     char *text;
     char url[URL_SIZE];