Move the function+line printing into GRE_DPRINTF().
Retire gre_closef(). Retire gre_join(). Constify gre_reconf(),
and don't pass it an LWP any longer.
Make this work in the new file descriptor regime. Add a kernel
thread per gre(4) instance whose purpose is to install the socket
into proc0's file descriptor table. Add gre_fp_send() and
gre_fp_recv() for passing file_t pointers to proc0.
Fix locking: don't solock() in the socket upcall, where it is
already held. Do solock() before calling soconnect().
Simplify reconfiguration.
Update a comment that mentions finding a less specific route, since
we don't do that any more.
5198e29b4c