int update_pic(User *user, const char *filename) {
    if (strlen(filename) > MAX_NAME - 1) {
        return 2;
    } else if (fopen(filename, "r") == NULL) {
        return 1;
    } else {
        strcpy(user->profile_pic, filename);
        return 0;
    }
}
Change the filename for the profile pic of the given user.
 *
 * Return:
 *   - 0 on success.
 *   - 1 if the file does not exist or cannot be opened.
 *   - 2 if the filename is too long.