dirmngr,w32: Hack around a select problem.

* dirmngr/dns.c (FD_SETSIZE): Bump up to 1024.
(dns_poll): Return an error instead of hitting an assertion failure.

For unknown reasons socket() return fd with values 244, 252, 268. The
latter is above the FD_SETSIZE of 256. It seems that select has been
build with a highler FD_SETSIZE limit. Bump up to a reasonable large
value.

A better solution would be to grab some code from npth_eselect to
replace select. We could also use npth_eselect direclty in
dns-stuff.c instead of using dns_res_poll.

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