make default pgp keylength konfigurable

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)

Unfortunately setting a bigger default might not be possible in many
cases, since servers usually don't have an overflowing random pool and
also timeouts in place which might interfere with prolonged key generation.

Therefore we leave the default to be 1024, but add a config switch to
let the admin decide, if a bigger keylength can be chosen in the
particular case.

Additionally add 4096 as a new option, since it is also commonly used.