make default pgp keylength 2048 and configurable

When users create new keys they usually don't consciously choose a
key length, but instead just keep the default one. Unfortunately the
current default of 1024 is not considered a sustainable choice anymore.
(http://en.wikipedia.org/wiki/Key_size#Asymmetric_algorithm_key_lengths)

Therefore I propose to set 2048 as the new standard and additionally make
the preselected value configurable, such that horde admins can consider
themselves if their current setup is efficient enough in gathering the
needed amount of randomness.
make the primary gpg keylength == the subkey length

Horde always generates 1024 bit primary keys. This should be considered
a bug, since by the semantic of all other key generating tools, key
length concerns primary and sub-key.
generate rsa/rsa keys instead of dsa

in accordance with gnupg we should create rsa keys instead of
deprecated dsa/el-gamal keys.

see:
http://lists.gnupg.org/pipermail/gnupg-devel/2009-May/025079.html
prefer sha-2 algos over sha-1 in key generation

following the new defaults of gpg.

see
http://lists.gnupg.org/pipermail/gnupg-announce/2009q3/000291.html
below "What's New"