Fix data sent to callback of hook_process, improve hook_process by using buffer

Fix: some data was sometimes missing (not sent to callback).
Improvement: use a 64KB buffer for child output and send data to callback only
when buffer is full.

doc/en/weechat_plugin_api.en.txt

@@ -6165,6 +6165,17 @@ Return value:
 
 * pointer to new hook, NULL if error occured
 
+[NOTE]
+Buffer size for sending data to callback is 64KB (there are 2 buffers: one for
+stdout and one for stderr).
+If output from child process (stdout or stderr) is longer than 64KB, callback
+will be called more than one time.
+
+[IMPORTANT]
+Even if most of times your callback is called only once, you must ensure that
+many calls to callback are ok in your code: you must concatenate data issued by
+many calls and use data only when return code is nonnegative.
+
 C example:
 
 [source,C]

doc/fr/weechat_plugin_api.fr.txt

@@ -6242,6 +6242,18 @@ Valeur de retour :
 
 * pointeur vers le nouveau "hook", NULL en cas d'erreur
 
+[NOTE]
+La taille du tampon pour l'envoi des données au "callback" est de 64 Ko (il y a
+2 tampons: un pour stdout et un pour stderr).
+Si la sortie du processus fils (stdout ou stderr) est plus longue que 64 Ko, le
+"callback" sera appelé plusieurs fois.
+
+[IMPORTANT]
+Même si la plupart du temps le "callback" n'est appelé qu'une seule fois, vous
+devez vous assurer que plusieurs appels au "callback" sont ok dans votre code :
+vous devez concaténer les données issues de plusieurs appels et n'utiliser les
+données que lorsque le code retour est positif ou nul.
+
 Exemple en C :
 
 [source,C]

doc/it/weechat_plugin_api.it.txt

@@ -6217,6 +6217,19 @@ Valore restituito:
 
 * puntatore al nuovo hook, NULL in caso di errore
 
+// TRANSLATION MISSING
+[NOTE]
+Buffer size for sending data to callback is 64KB (there are 2 buffers: one for
+stdout and one for stderr).
+If output from child process (stdout or stderr) is longer than 64KB, callback
+will be called more than one time.
+
+// TRANSLATION MISSING
+[IMPORTANT]
+Even if most of times your callback is called only once, you must ensure that
+many calls to callback are ok in your code: you must concatenate data issued by
+many calls and use data only when return code is nonnegative.
+
 Esempio in C:
 
 [source,C]