Add -?, -help, and --help as alternative options to get help.

Originally committed as revision 20906 to svn://svn.ffmpeg.org/ffmpeg/trunk

cmdutils_common_opts.h

     { "L", OPT_EXIT, {(void*)show_license}, "show license" },
     { "h", OPT_EXIT, {(void*)show_help}, "show help" },
+    { "?", OPT_EXIT, {(void*)show_help}, "show help" },
+    { "help", OPT_EXIT, {(void*)show_help}, "show help" },
+    { "-help", OPT_EXIT, {(void*)show_help}, "show help" },
     { "version", OPT_EXIT, {(void*)show_version}, "show version" },
     { "formats"  , OPT_EXIT, {(void*)show_formats  }, "show available formats" },
     { "codecs"   , OPT_EXIT, {(void*)show_codecs   }, "show available codecs" },

doc/ffmpeg-doc.texi

@@ -228,7 +228,7 @@ specified for the inputs.
 @item -L
 Show license.
 
-@item -h
+@item -h, -?, -help, --help
 Show help.
 
 @item -version

doc/ffplay-doc.texi

@@ -29,7 +29,7 @@ ffplay [options] @file{input_file}
 @section Main options
 
 @table @option
-@item -h
+@item -h, -?, -help, --help
 Show help.
 @item -version
 Show version.

doc/ffserver-doc.texi

@@ -251,7 +251,7 @@ Show available bitstream filters.
 Show available protocols.
 @item -filters
 Show available libavfilter filters.
-@item -h
+@item -h, -?, -help, --help
 Show help.
 @item -loglevel @var{loglevel}
 Set the logging level used by the library.