dirmngr: Use a function to increment network short.

* dirmngr/dns.c (plus1_ns): New.
(dns_p_push): Use it.

On OpenBSD, htons and ntohs are expanded to GCC's statement
expressions where local variable is allowed. Consecutive use
of htons and ntohs causes problem of variable name.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>