Issue #31, Add commands verify-keys and verify-requests

This commit splits the command verify into verify-keys and
verify-requests. The arguments “keys” and “requests” were non-optional
arguments for verify anyway. This is more consistent.

This will also add -server <server> as optional option for both verify
commands, usable as optional filter.

Signed-off-by: Lars Kiesow <lkiesow@uos.de>