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 are part of this PR:

• 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>

• make tests, httpserver-api, and some other modules compilable with non-C++ 11 level
  Please note, that all the unit tests and modules implemented in C++ are not subject to this change. On that note, the unit tests are passing.

An example of how to build with C++ 17:

./scripts/build image=httpserver-api fs=rofs.fg -j$(nproc) conf_cxx_level=gnu++17

Finally, this PR does not change the default C++ level which remains at 11, until we permanently decide to change in the future.

[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)

[wkozaczuk]

@nyh I have updated this PR. The changes include fixes to core/callstack.cc and adding new build config option conf_cxx_level. So the kernel and all relevant modules get built with non-c++_11 only if explicitly specified.