MFC: r310712

- Use correct offsets into the keys set array. As the elements of this
zero-length array are dynamically sized at run-time based on the use
of hints, compilers can't be expected to figure out these offsets on
their own. [1]
- Fix incorrect comparison in cmp_nans(). [2]

PR: 204571 [1], 202301 [2]
Submitted by: David Binderman [2]

MFC r312667:
sort - Don't live-loop threads.

Worker threads now use a pthread_cond_t to wait for work instead of
burning the cpu up.

Obtained from: DragonflyBSD (07774aea0ccf64a48fcfad8899e3bf7c8f18277a)

MFC r259740:

In usr.bin/sort/radixsort.c, pop_ls_mt() is only referenced if
SORT_THREADS is defined, so make the whole function conditional, instead
of just the pthread calls in it.

MFC r259740:

In usr.bin/sort/radixsort.c, pop_ls_mt() is only referenced if
SORT_THREADS is defined, so make the whole function conditional, instead
of just the pthread calls in it.