python: default op_keylist_start parameters.

* lang/python/gpgme.i: Added gpgme_op_keylist_start with defaults
* lang/python/tests/t-keylist.py: Added tests for default parameters

To increase the ease of use, op_keylist_start
parameters default to sensible values.
The empty string matches all keys.
We assume that the user wants to retrieve public keys most of the time,
so we default to public keys rather than secret keys.

• Signed-off-by: Tobias Mueller <muelli@cryptobitch.de>