Fix an assortment of dupprintf() format string bugs.

I've enabled gcc's format-string checking on dupprintf, by declaring
it in misc.h to have the appropriate GNU-specific attribute. This
pointed out a selection of warnings, which I've fixed.

[originally from svn r10084]

unix/uxpty.c

@@ -938,7 +938,7 @@ static const char *pty_init(void *frontend, void **backend_handle, Conf *conf,
 
     *backend_handle = pty;
 
-    *realhost = dupprintf("\0");
+    *realhost = dupstr("");
 
     return NULL;
 }