Make `tripe' be the default key type.

Make `tripe' be the default key type.

Rather than using the key type to tell us which key-exchange group type
to use, read it out of an attribute on the key.  This makes the whole
finding keys thing much simpler to explain (although the code becomes
slightly more complicated).  Fall back to the old behaviour of parsing
the group type out of the key type if there's no attribute.

Reorganize the tripe(8) manual page somewhat.  Remove the tutorial-ish
part, which is now out of place and not very useful; there's a proper
Texinfo manual coming soon.