add a recurring help message for the first 3 missions

Following a good suggestion from @leroivi

during the first 3 missions, each prompt is preceeded by a line
[use 'gsh help' to get a list of available commands]
so that people who've never used a shell don't forget about that.

This is disabled when the "quiet" option is given ("-q"), or when
gameshell is run in debug mode ("-d" or "-D").

(If needs be, this can be deactivated by setting the "GSH_HELP_HINT"
variable to "never"; and can be kept beyond the first 3 missions by
setting the "GSH_HELP_HINT" variable to "always".)

i18n/en.po

@@ -294,5 +294,8 @@ msgstr  "test $nb_tests failed"
 msgid   "total time: %s"
 msgstr  "total time: %s"
 
+msgid   "use 'gsh help' to get a list of available commands"
+msgstr  "use 'gsh help' to get a list of available commands"
+
 msgid   "y"
 msgstr  "y"

i18n/fr.po

@@ -301,10 +301,9 @@ msgstr  "test $nb_tests échoué"
 msgid   "total time: %s"
 msgstr  "temps total : %s"
 
+msgid   "use 'gsh help' to get a list of available commands"
+msgstr  "utilisez 'gsh help' pour obtenir une liste de commandes possibles"
+
 # abbreviation for "yes" (lowercase)
 msgid   "y"
 msgstr  "o"
-
-#, fuzzy
-#~ msgid        "You shouldn't run Gameshell as root!"
-#~ msgstr       "Erreur: il ne faut pas lancer Gameshell avec l'utilisateur root !"