daemon: fix cleanup of stringsbuf usages

Declare most of the stringsbuf as CLEANUP_FREE_STRINGSBUF, so they are
freed completely on stack unwind: use take_stringsbuf() in return
places to take away from the stringsbuf its content, and remove all the
manual calls to free_stringslen (no more needed now).
This requires to not use free_stringslen anymore on failure in the
helper functions of stringsbuf, which now leave the content as-is (might
be still useful even on error).

This allows us to simplify the memory management of stringsbuf's, which
are not properly fully freed, fixing memory leaks in some error paths
(which were not calling free_stringslen).

daemon/9p.c

@@ -41,7 +41,7 @@ static char *read_whole_file (const char *filename);
 char **
 do_list_9p (void)
 {
-  DECLARE_STRINGSBUF (r);
+  CLEANUP_FREE_STRINGSBUF DECLARE_STRINGSBUF (r);
 
   DIR *dir;
 
@@ -60,7 +60,7 @@ do_list_9p (void)
     if (end_stringsbuf (&r) == -1)
       return NULL;
 
-    return r.argv;
+    return take_stringsbuf (&r);
   }
 
   while (1) {
@@ -93,7 +93,6 @@ do_list_9p (void)
   /* Check readdir didn't fail */
   if (errno != 0) {
     reply_with_perror ("readdir: /sys/block");
-    free_stringslen (r.argv, r.size);
     closedir (dir);
     return NULL;
   }
@@ -101,7 +100,6 @@ do_list_9p (void)
   /* Close the directory handle */
   if (closedir (dir) == -1) {
     reply_with_perror ("closedir: /sys/block");
-    free_stringslen (r.argv, r.size);
     return NULL;
   }
 
@@ -113,7 +111,7 @@ do_list_9p (void)
   if (end_stringsbuf (&r) == -1)
     return NULL;
 
-  return r.argv;
+  return take_stringsbuf (&r);
 }
 
 /* Read whole file into dynamically allocated array.  If there is an