Fix tincd logfile location when running tests.
Clean up leftover tincd and sptps_test processes.
Send a RELOAD to a running tincd when a new invitation key has been generated.
Slightly relax the connection rate limit for a single address.

The restriction of accepting only 1 connection per second from a single address
is a bit too much, especially if one wants to join a VPN using an invitation,
which requires two connections.
Also test whether tinc daemons can connect to each other after import/export.
Add a test for invite and join commands.