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/access/transam/xlogarchive.c

@@ -33,11 +33,11 @@
  * Attempt to retrieve the specified file from off-line archival storage.
  * If successful, fill "path" with its complete path (note that this will be
  * a temp file name that doesn't follow the normal naming convention), and
- * return TRUE.
+ * return true.
  *
  * If not successful, fill "path" with the name of the normal on-line file
  * (which may or may not actually exist, but we'll try to use it), and return
- * FALSE.
+ * false.
  *
  * For fixed-size files, the caller may pass the expected size as an
  * additional crosscheck on successful recovery.  If the file size is not