build: Add -Werror=unused... compile flags

With C++17 [[maybe_unused]] attribute it is easy to keep emerged cases in the new code under control.

This PR does not touch -Wno-unused-parameter as it requires too much of code churning.

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Compiler specific notes:

1. clang:

• -Wunused-macros not added because it requires a non-trivial checking in the configure script, and it blames only https://github.com/bitcoin/bitcoin/blob/a9d1b40d53ec417eefbe767aa66701ef8e1801d5/src/util/system.cpp#L34
• -Wunused-template not added because some of our templates are buried into macros
• -Wused-but-marked-unused produces tones of warnings in boost tests

2. gcc

• -Werror=unused-const-variable with level 2 seems not suitable for C++ code