1. Use `const char *flag_desc` because constructing a SCP_string for each flag description is not needed
2. Make `VALID_FNAME` check for nullptr
3. Modify the signature of `coalesce()` to allow capturing of mixed `const` and non-`const` arguments
4. Use `std::for_each` (via `SCP_tolower`) rather than `std::transform` to fix compilation warnings