If IPO is enabled, MSVC compiles with /GL which is not
compatible with cmake's WINDOWS_EXPORT_ALL_SYMBOLS.
Since server.dll can't autoexport symbols from /GL compiled objects,
IPO must be disabled for dbug, string,mysys, and sql targets (in case we
build "plugin" DLLs that need server.dll's symbols)
Removed the existing nt_service classes - they provide little
abstraction, and only obscure a relatively simple service handling.
This replaces by similar code inspired by MS docs samples.
Service handling is now moved into winmain.cc, which contains
the main() function for Windows.
winmain provides reporting callbacks, which should be used by external code
,to report transitions from starting to running to shutting down to stopped.
Removed a do-nothing ServiceMain thread, and the
non-working service "pause/continue". Removed a lot of #ifdef __WIN__
code from mysqld.cc
