dirmngr: Do not use MAXDNAME.

* dirmngr/dns-stuff.c (getsrv): Replace MAXDNAME.
* dirmngr/dns-stuff.h (MAXDNAME): Remove.
(struct srventry): Use a fixed value instead of MAXDNAME.
* dirmngr/http.c (connect_server): Use DIMof instead of MAXDNAME.
Malloc a helper array.

Depending on the order of included headers it might be that we allocate
the array with a different size than what we test against in another
module. To make it more robust we use the actual known size of
checking.

A better would be to use a linked list and avoid these large arrays.

• Signed-off-by: Werner Koch <wk@gnupg.org>