maint: Syntax cleanup

* dietncl/filter/import.lua: Likewise.
* dietncl/filter/prenorm.lua: Likewise.
maint: Update copyright year

* Makefile: Likewise.
* README: Likewise.
* TODO: Likewise.
* dietncl/filter/import.lua: Likewise.
* dietncl/filter/prenorm.lua: Likewise.
* dietncl/filter/region.lua: Likewise.
* dietncl/filter/transition.lua: Likewise.
* dietncl/init.lua: Likewise.
* dietncl/nclaux.lua: Likewise.
* dietncl/path.lua: Likewise.
* dietncl/xmlsugar.lua: Likewise.
* tests/test-filter-import.lua: Likewise.
* tests/test-filter-prenorm.lua: Likewise.
* tests/test-filter-region.lua: Likewise.
* tests/test-filter-transition.lua: Likewise.
* tests/test-init-parse.lua: Likewise.
* tests/test-init-parsestring.lua: Likewise.
* tests/test-nclaux-gen-id.lua: Likewise.
* tests/test-nclaux-timetoseconds.lua: Likewise.
* tests/test-path-absolute.lua: Likewise.
* tests/test-path-join.lua: Likewise.
* tests/test-path-relative.lua: Likewise.
* tests/test-path-split.lua: Likewise.
* tests/test-xmlsugar-attributes.lua: Likewise.
* tests/test-xmlsugar-children.lua: Likewise.
* tests/test-xmlsugar-clone.lua: Likewise.
* tests/test-xmlsugar-equal.lua: Likewise.
* tests/test-xmlsugar-eval.lua: Likewise.
* tests/test-xmlsugar-gmatch.lua: Likewise.
* tests/test-xmlsugar-insert.lua: Likewise.
* tests/test-xmlsugar-match.lua: Likewise.
* tests/test-xmlsugar-remove.lua: Likewise.
* tests/test-xmlsugar-replace.lua: Likewise.
* tests/test-xmlsugar-userdata.lua: Likewise.
* tests/util.lua: Likewise.
build: Use relative LUA_PATH in Makefile

* Makefile: Remove unused variables; clean up syntax.
(LUA_PATH): Use relative paths.