Use real number of cores for default -par, ignore virtual cores

To determine the default for -par, the number of script verification
threads, use boost::thread::physical_concurrency()
which counts only physical cores, not virtual cores.

Virtual cores are roughly a set of cached registers to avoid context
switches while threading, they cannot actually perform work, so spawning
a verification thread for them could even reduce efficiency and will put
undue load on the system.

Should fix issue #6358, as well as some other reported system overload
issues, especially on Intel processors.

The function was only introduced in boost 1.56, so provide a utility
function GetNumCores to fall back for older Boost versions.