This is for backwards-compatibility with older .inx files that used
to redundantly request a dependency on their own interpreted script
and did so with type="executable".

However, in practice script files often *don't* have the x-bit set,
which fortunately did not cause problems in the past, though, as the
check for the x-bit was broken, see
  3da7f71e45eb986aef67771b5af3c1e308971cff

For "executable" files only existence was checked.
(Glib::file_test needs a single FileTest)

Apply this properly when checking script <command>s and xslt <file>s
- interpreted scripts and xslt files -> only check existence
- un-interpreted scripts -> check for executable file

For Windows workarounds are implemented to yield desirable behavior:
- as there is no executable bit, only check existence
- as executables usually come with an extensions, scan for those
  as well.

- Use identical logic for looking up <dependency>s and <command>s.
- Remove duplicate (but inconsistent and incomplete) logic from
  script.cpp that was used to search for the <command> again
- Remove <check> element from .inx format
  It seems unused (at the very least by core extensions) and
  redundant to <dependency> checking
- Deprecate the <command>-specific "reldir" attribute.
  Consistently use the functionally identical "location" attribute
  that was only used for <dependency>s before
- Introduce the new relative location value location="inx",
  which looks up <dependencies> and <command>s relative to the
  .inx file's location.

(std::ostream is not properly portable unfortunately)

- add license headers to everything
- convert a few files from public domain or LGPL2.1+ to GPL2+
- some archaeology to clarify which files are from which library

Found via `codespell`

This replaces all NULL or 0 with nullptr when assigned to or returned as
a pointer.

This reverts commit bba4ff6672494fab59286b50f6c645ad62a47e60.