Improves readability of the log files.

* pkg-config check for `ncurses` fails if it failed previously for `ncursesw`.
  This is the case e.g. for ncurses from NetBSD's pkgsrc.
* No longer assume that any libncurses is not enhanced (X/Open compatible).
* SciTECO and Scinterm require to find a curses.h in the include paths.
  The ncurses check must therefore not be limited to the first best
  ncurses/ncurses.h and the like.
* We now always check for X/Open compatibility and always require
  a curses.h in the standard directories or as given by pkg-config.
* AX_WITH_CURSES was radically rewritten and is now called AX_WITH_NCURSES.
* --with-interface=netbsd-curses gets its own detection code.
  It always requires a curses.h in the standard paths and a libcurses.
  It should now be fixed for real NetBSD installations if the ncurses
  port is installed as well.
* Unified all of the curses-arguments to CURSES_CFLAGS and CURSES_LIBS.
  There is no reason we need PDCURSES_CFLAGS, XCURSES_CFLAGS etc.

Ther rest of mingw32 still appears to exist, though.

* We can therefore no longer provide 20.04 nightly builds.
  Perhaps I will manually build binary releases for the v2.4.0 release for the last time.
  The PPA will still provide 20.04 of course.
* The AppImages are consequently also built based on the Ubuntu 22.04 packages,
  which are now the oldest supported ones.

* `--enable-html-manual` renamed to `--enable-html-docs`.
* It's also uploaded to the website and linked to in README.

* Also run CI on 24.04.
* The Ubuntu 20.04 runner is deprecated soon until 1. April 2025,
  but for the time being we keep supporting it as well.

It can be done only under ncurses, as Gtk results in many false positives.
Also, try to use it on GCC and Clang.
It didn't work with GCC on FreeBSD, but perhaps it will work on Ubuntu.

As I cannot run the test suite with Valgrind in Github runners,
this should still catch some memory bugs during test suite runs.