MEDIUM: ssl: Add standardized DH parameters >= 1024 bits

This patch adds standardized (rfc 2409 / rfc 3526)
DH parameters with prime lengths of 1024, 2048, 3072, 4096, 6144 and
8192 bits, based on the private key size.
BUILD/MINOR: ssl: remove one call to sprintf()

Lukas reported another OpenBSD complaint about this use of sprintf() that
I missed :

src/ssl_sock.o(.text+0x2a79): In function `bind_parse_crt':
src/ssl_sock.c:3015: warning: sprintf() is often misused, please use snprintf()

This one was even easier to handle. Note that some of these calls could
be simplified by checking the snprintf output size instead of doing the
preliminary size computation.