Stop inlining the struct ucond definition into struct pthread_cond.
This avoids unneccessary casts and make the calls to _thr_ucond_*()
functions less questionable.

The c_spare field was not included into struct pthread_cond, so the
change modifies libthr ABI for shared condvars. But since an off-page
does not legitimately contains any other data past the struct
pthread_cond, the change keeps shared condvars from pre- and post-
changed libthr compatible. Also note that the whole struct ucond was
never copied in or out by kernel.

For private condvars, the privately allocated memory was never exposed
outside libthr.

Sponsored by: The FreeBSD Foundation