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.