GnuTLS: use 1024 (DH_BITS_MIN) as minimum size of the DH prime

For outgoing connections, we use 2048 (DH_BITS) since commit 49b2d0e.

This patch enables ngIRCd to accept incoming connections from other servers
and clients that use at least 1024 bits (and no longer requires 2048 for
incoming connections, too).

Patch proposed by Florian Westphal.