Removing MSVC C4100 from pragma block at the top of pybind11.h (#3150)

* Removing pragma for 4100 (to see what is still broken with the latest code).

* Adding --keep-going

* Revert "Adding --keep-going"

This reverts commit 1c844c6ffd07a6111b644811e7e3b0a50b9d44bb.

* Introducing PYBIND11_WORKAROUND_INCORRECT_MSVC_C4100.

* _MSC_VER <= 1916

* Replacing simple variadic function with variadic template (attempt to resolve MSVC 2017 failures).

* Preserving existing comment (moved from pybind11.h to detail/common.h).

* Adding blank lines for readability.