Update places that mentioned "--fullscreen" with no args

Since `parse.gperf` was added long ago, the ability to run Tux Paint
with "--fullscreen" command-line, or simply "fullscreen" in the
config. file, no longer exists.

It must be `--fullscreen=yes` (or `--fullscreen=native` as an
alternative to `--fullscreen=yes --native`).  It can also accept
`--fullscreen=no` as an alternative to `--windowed`.

Updating the following places

 * BASH tab completion
 * `tuxpaint --help` output
 * FAQ

h/t Karl for reporting

Closes https://sourceforge.net/p/tuxpaint/bugs/289/

docs/CHANGES.txt

@@ -133,6 +133,13 @@ https://tuxpaint.org/
       Advanced Stamps Howto, as well.
       Bill Kendrick <bill@newbreedsoftware.com>
 
+    * Correct places where we still said `--fullscreen`
+      (stand-alone, with no "yes", "no", or "native" argument)
+      was allowed.
+      Closes https://sourceforge.net/p/tuxpaint/bugs/289/
+      (h/t Karl Ove Hufthammer for reporting)
+      Bill Kendrick <bill@newbreedsoftware.com>
+
   * Localization Updates:
     ---------------------
     * Bulgarian translation