nproc: denote an incompatiblity with Linux

On Linux _NPROCESSORS_CONF reports CPU threads disabled by the kernel,
while it does not on FreeBSD.

Flip _NPROCESSORS_ONLN to _NPROCESSORS_CONF. While it keeps reporting
the same value, it will automagically unbreak should someone change the
above.

Approved by: re (cperciva)
(cherry picked from commit 059320b8c8de73a90514879cd1e2da397584cfde)
(cherry picked from commit 117d4cafdcb806407e38266de44aebe0b4e62b6f)