several utility enhancements

[Goober5000]

These were coded for the split_str refactor, but are generally useful elsewhere as well.

1. Use SCP_string in some utility functions where appropriate
2. Avoid copying C-strings in some utility functions where it isn't necessary
3. Add concat functions to SCP_vector
4. Change return type int to bool on some common parselo functions
5. Add find_white_space and find_gray_space parselo functions

Depends on #7097; in draft until that is merged.