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/

src/tuxpaint-completion.bash

@@ -3,7 +3,7 @@
 # Bill Kendrick <bill@newbreedsoftware.com>; https://tuxpaint.org/
 # Based on inkscape's completion file, by allali@univ-mlv.fr
 #
-# Last modified 2024-04-10
+# Last modified 2024-06-02
 #
 # $Id$
 
@@ -31,8 +31,9 @@ _tuxpaint()
 			-c --copying \
 			-u --usage \
 			--listfonts \
-			-w --windowed -f --fullscreen \
-			--native \
+			-f=yes --fullscreen=yes \
+			--fullscreen=native --native \
+			-w --windowed -f=no --fullscreen=no \
 			--disablescreensaver --allowscreensaver \
 			--orient=landscape --orient=portrait \
 			--buttonsize --buttonsize=48 --buttonsize=auto \

src/tuxpaint.c

@@ -8109,13 +8109,14 @@ void show_usage(int exitcode)
           "  [--nosysconfig]\n"
           "\n"
           " Video/Sound:\n"
-          "  [--windowed | --fullscreen]\n"
+          "  [--fullscreen=yes | --fullscreen=native | --fullscreen=no]\n"
+          "  [--windowed]\n"
           "  [--WIDTHxHEIGHT | --native]\n"
           "  [--orient=landscape | --orient=portrait]\n"
           "  [--disablescreensaver | --allowscreensaver ]\n"
           "  [--sound | --nosound]\n"
           "  [--stereo | --nostereo]\n"
-          "  [--buttonsize=N] (24-192; default=48) | [--buttonsize=auto]\n"
+          "  [--buttonsize=N (24-192; default=48) | --buttonsize=auto]\n"
           "  [--colorsrows=N] (1-3; default=1)\n"
           "  [--colorfile FILE]\n"
           "\n"