cmake: fix broken dependency chain for cmdline-opts, tidy-ups
- make `curl.1` and `curl.txt` depend on `DPAGES`. To trigger a rebuild when an individual manpage is updated. - tell CMake that the cmdline-opts command also creates `curl.txt`. - make `tool_hugehelp.c` depend on `curl.txt` (was: `curl.1`), to match what it actually uses for input. - stop using `generate-curl.1` as an indirect way to create `curl.txt` in time for `tool_hugehelp.c`. After the fixes above there is a direct depedency chain between them. - move `ASCIIPAGE` and `MANPAGE` variables to top-level, re-use them in `src` and prefix them with `CURL_` to avoid clashing with other projects. - drop double quotes from `generate-curl.1` as a hint that it is not a filename, but a target name. - src: tidy up order of dependency lists. Closes #14883