Use nproc(1) instead of sysctl kern.smp.cpus

[mjg]

Summary

The former respects cpuset, while the latter does not.

$ cpuset -l 1 -- make -V MAKE_JOBS_NUMBER
1

As nproc is newly introduced, keep the previous variant as a fallback for the time being.

[mandree]

This comment was removed by mandree.

[bdrewery]

Mk/bsd.port.mk

2044

This should be ${NPROC} and a new NPROC?= line added in Mk/bsd.commands.mk.

[mjg]

• NPROC

[bdrewery]

I didn't mean to "accept as portmgr".

[mandree]

Note this introduces an extra PATH search for systems that do NOT yet have nproc so perhaps we need to add some OSVERSION switch here.