common: New function cmp_canon_sexp.

* common/sexputil.c (cmp_canon_sexp): New.
(cmp_canon_sexp_def_tcmp): New.
* common/t-sexputil.c (test_cmp_canon_sexp): Add a simple test.

To be used to fix

(cherry picked from commit b6ba6a79ce9336f1b53f16f3d1190dd009fb166e)

• GnuPG-bug-id: T5061
• Signed-off-by: Werner Koch <wk@gnupg.org>