This patch adjusts the bsd-uthread target to use thread_change_ptid,
and to never let the core see an event happen in a thread not in
the thread table.

Tested on i386-unknown-freebsd6.0 with a little hack in the
testsuite to link the gdb.thread tests against -lc_r instead
of -lpthread, and on i386-unknown-openbsd4.3, where no hack is
needed.

This patch depends on patch 1 and 2.

OK?

-- 
Pedro Alves