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.

(cherry picked from commit 059320b8c8de73a90514879cd1e2da397584cfde)