Add posix_tnode to <search.h>.

Add posix_tnode to <search.h>.

In r307227 I've refactored the binary search tree functions to use the
posix_tnode type. As this change does not apply cleanly to this version
of FreeBSD, only make the change that matters: add the definition of the
newly introduced type.

This will ease source-level compatibility going forward.

git-svn-id: svn://svn.freebsd.org/base/stable/10@308091 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f