- 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.

- 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

This renames most C <*.h> includes into C++ <c*> includes.

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

This reverts commit bba4ff6672494fab59286b50f6c645ad62a47e60.

(bzr r15035)

(bzr r15034)

(bzr r15027)