pgsql: Rewrite LIKE's %-followed-by-_ optimization so it really works

Log Message:
-----------
Rewrite LIKE's %-followed-by-_ optimization so it really works (this time
for sure ;-)). It now also optimizes more cases, such as %_%_. Improve
comments too. Per bug #5478.

In passing, also rename the TCHAR macro to GETCHAR, because pgindent is
messing with the formatting of the former (apparently it now thinks TCHAR
is a typedef name).

Back-patch to 8.3, where the bug was introduced.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
like_match.c (r1.27 -> r1.28)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/like_match.c?r1=1.27&r2=1.28)
pgsql/src/test/regress/expected:
strings.out (r1.40 -> r1.41)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/strings.out?r1=1.40&r2=1.41)
pgsql/src/test/regress/sql:
strings.sql (r1.28 -> r1.29)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/strings.sql?r1=1.28&r2=1.29)