Coexist with quilt: rename PATCHES --> PATCHES.txt

The issues is on Win32, which ignores case in filesystem
and thus doesn't tolerate the quilt "patches" directory.

Rename, and add "patches" to .gitignore so that developers
can choose to use quilt for local patch management.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

Doxyfile.in

@@ -567,7 +567,7 @@ WARN_LOGFILE           =
 INPUT                  = @srcdir@/doc/manual \
                          @srcdir@/TODO \
                          @srcdir@/BUGS \
-                         @srcdir@/PATCHES \
+                         @srcdir@/PATCHES.txt \
                          @srcdir@/src \
                          @srcdir@/config.h