Progress: Various changes and fixes.

Progress: Various changes and fixes.

I decided that I needed to expand the `f_fss_skip_past_space()` into `f_fss_skip_past_space_previous()` to expose the `previous` position variable to the caller.
I must avoid breaking the **API**/**ABI**, so `f_fss_skip_past_space()` is not changed.
This should simplify the embedded read functions.

Fix documentation, adding "Must not be NULL." where it is missing.

The man pages will need to be re-generated more than likely.
Just copy the `f_fss_skip_past_space()` man page and update it manually for now.

Add missing `_di_f_utf_is_zero_width_` check on `private_f_utf_character_is_joiner()`.