Change TRUE/FALSE to true/false

The lower case spellings are C and C++ standard and are used in most
parts of the PostgreSQL sources.  The upper case spellings are only used
in some files/modules.  So standardize on the standard spellings.

The APIs for ICU, Perl, and Windows define their own TRUE and FALSE, so
those are left as is when using those APIs.

In code comments, we use the lower-case spelling for the C concepts and
keep the upper-case spelling for the SQL concepts.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

src/backend/utils/time/combocid.c

@@ -142,8 +142,8 @@ HeapTupleHeaderGetCmax(HeapTupleHeader tup)
  * into its t_cid field.
  *
  * If we don't need a combo CID, *cmax is unchanged and *iscombo is set to
- * FALSE.  If we do need one, *cmax is replaced by a combo CID and *iscombo
- * is set to TRUE.
+ * false.  If we do need one, *cmax is replaced by a combo CID and *iscombo
+ * is set to true.
  *
  * The reason this is separate from the actual HeapTupleHeaderSetCmax()
  * operation is that this could fail due to out-of-memory conditions.  Hence

src/backend/utils/time/tqual.c

@@ -1422,7 +1422,7 @@ HeapTupleSatisfiesNonVacuumable(HeapTuple htup, Snapshot snapshot,
  *	should already be set.  We assume that if no hint bits are set, the xmin
  *	or xmax transaction is still running.  This is therefore faster than
  *	HeapTupleSatisfiesVacuum, because we don't consult PGXACT nor CLOG.
- *	It's okay to return FALSE when in doubt, but we must return TRUE only
+ *	It's okay to return false when in doubt, but we must return true only
  *	if the tuple is removable.
  */
 bool