tags: unify recursion for tags targets with that of "usual" targets

tags: unify recursion for tags targets with that of "usual" targets

This change has a side effect: now any failure of the 'tags', 'ctags',
'cscope' and 'cscopelist' targets in a subdirectory will cause the
recursive command issued from the top directory to fail.  We believe
that this behavioural change is not only justified by the simplification
this patch entails, but actually offers better semantics: ignoring
failures by default is never a good idea, and with make, the user can
anyway order the completion of commands in the face of possible failures
with the '-k' option.

* NEWS: Update.
* automake.in (%required_targets): Add keys 'cscopelist-am', 'tags-am'
and 'ctags-am'.
(handle_tags): Greatly simplified, the major part of the handling of
recursion for the 'tags', 'ctags' and 'cscopelist' now moved out to ...
* lib/am/tags.am: ... this file, that has been adjusted and extended
accordingly.
* t/maken3.sh: Remove testing of the 'TAGS' target, that is now just
an alias to 'tags'.  Without this change, this test would spuriously
fail.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>