pgsql: Minor cleanup/code review for "indirect toast" stuff.

Minor cleanup/code review for "indirect toast" stuff.

Fix some issues I noticed while fooling with an extension to allow an
additional kind of toast pointer.  Much of this is just comment
improvement, but there are a couple of actual bugs, which might or might
not be reachable today depending on what can happen during logical
decoding.  An example is that toast_flatten_tuple() failed to cover the
possibility of an indirection pointer in its input.  Back-patch to 9.4
just in case that is reachable now.

In HEAD, also correct some really minor issues with recent compression
reorganization, such as dangerously underparenthesized macros.

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/ccf655c446d9b2de98455cf29f27795e0e067779

Modified Files
--------------
src/backend/access/heap/tuptoaster.c |   53 ++++++++++++++++++++++++----------
src/include/access/tuptoaster.h      |    6 ++--
src/include/postgres.h               |   37 ++++++++++++------------
3 files changed, 59 insertions(+), 37 deletions(-)