pgsql: Fix bogus tree-flattening logic in QTNTernary().

Fix bogus tree-flattening logic in QTNTernary().

QTNTernary() contains logic to flatten, eg, '(a & b) & c' into 'a & b & c',
which is all well and good, but it tries to do that to NOT nodes as well,
so that '!!a' gets changed to '!a'. Explicitly restrict the conversion to
be done only on AND and OR nodes, and add a test case illustrating the bug.

In passing, provide some comments for the sadly naked functions in
tsquery_util.c, and simplify some baroque logic in QTNFree(), which
I think may have been leaking some items it intended to free.

Noted while investigating a complaint from Andreas Seltenreich.
Back-patch to all supported versions.

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/2a2b439cc14288c539b4b875df275766845c4c99

Modified Files
--------------
src/backend/utils/adt/tsquery_util.c | 82 +++++++++++++++++++++++++----------
src/test/regress/expected/tsearch.out | 7 +++
src/test/regress/sql/tsearch.sql | 2 +
3 files changed, 68 insertions(+), 23 deletions(-)