Longer default PKCS keys

This commit changes the default PKCS key length from 2048 bits to 4096
bits.  It adds warnings to both SILC client and SILC server in start up
in case the existing key is shorter than 4096 to encourage people to
generate new key longer key pair.

This commit also changes the default SKE DH group from 1024 to 1536 bits.
The old group is still supported.