tinc 1.1pre support: Starting with current git (and 1.1pre7) the binary
that was called 'tincctl' is now just called 'tinc'. So change the default
value.

The name of the used variables are kept at "tincctl" for now to avoid
confusion.

If you are still using an older tinc 1.1pre version you can change back
to the previous binary name by using new configuration variable
$tincctl_bin