enhance utility functions

1. Add optional `start_index` and `num_items` parameters to the find/count functions in utility.h.
2. Fix an accidental recursion bug.
3. Change appropriate parameters to `size_t` and cast appropriately.
4. Add two forms of `SCP_vector_contains_lcase`.
5. Enhance `sprintf` and add `sprintf_concat` for `SCP_string`.

Author: Goober5000