kernel: upgrade to C++ 17

[wkozaczuk]

This patch makes necessary changes to the relevant source files to make it possible to build kernel at C++ 17 level. This will allow us to take advantage of new features available in C++ 17 standard.

The following changes have been:

• remove obsolete register keyword (mostly under bsd/ part of the tree)

• add const specifier to the comparison operator definitions where necessary

• replace deprecated std::iterator usage with explicit definitions of traits (for example iterator_category, value_type, and 3 others) (for some background info read this - https://www.fluentcpp.com/2018/05/08/std-iterator-deprecated/)

• add constexpr to the friend declaration of std::distance<mbuf_iterator>

Please note all the unit tests and modules implemented in C++ are not subject to this change at this point. On that note, the unit tests seem to be passing.

TODO: Fix compilation errors in core/callstack.cc. The kernel does not seem to depend on it as I temporarily disabled it in makefile. The only place that uses it is modules/java-mgmt/jni/tracepoint.cc. I could not figure out how to fix the compilation errors.

[nyh]

I'm curious, what is the motivation of this change. Which C++17 features are you planning to use?
But I admit it's kind of silly to stick to the 12 year old C++11 standard, so I won't oppose this change.

[wkozaczuk]

@nyh I have some ideas on how to tackle #853 and I needed a way to re-purpose existing waitlist objects and instead of deleting them, keep them (your idea to comment out erase) and "re-key" unused entries by doing extract and insert (see https://www.fluentcpp.com/2020/05/01/how-to-change-a-key-in-a-map-or-set-in-cpp/). This is to minimize allocations.

[wkozaczuk]

I have relied on the compiler to tell me what is wrong and then verify things still work by using unit tests. But based on your experience are there potentially things that may get broken by this change and we would not even know about it? (see 'Changes to evaluation order' or 'Guaranteed copy elision' in https://developers.redhat.com/articles/2021/08/06/porting-your-code-c17-gcc-11)