Keyset file location vs name

[AdamISZ]

The code currently reads the keysets config var as context_label:filepath but this is dumb: we want the second part of that string to represent the name of the keyset, shared by client and server (or peer and peer), whereas the location on disk is a different thing. We could try to force that to be the same for both parties using a default directory location for keysets, but it seems unnecessary. Instead, we'll add another config var with an easy default for the location of keysets, and then include only the name (the *.aks string as described here) in the keysets string.