[NFC] Use C++ features and headers in C++ code instead of C

[gfunni234_gmail.com]

Summary

We don’t need to use integers as booleans or the C standard headers when we have the C++ standard headers and booleans

[jrtc27]

It's written this way because our C++ is deliberately very C-like. And it avoids all the std:: everywhere that's just a mess. Are there any useful changes you're making here other than changing it to match your personal views on style? The only somewhat useful thing I can see is changing some ints to bools, but even then it's all in tests so I really don't care all that much about a tiny increase in memory footprint.

usr.sbin/pmc/cmd_pmc_summary.cc

184

It's already applied to the prototype, just like all the other cmd_pmc_*