[WIP] Refactoring platform-specific code in util.h/util.cpp #13209

Take all of the functions that are specific to Windows or Unix, and create a common set of methods in a new header file, all combined in the src/util/ folder.

The goal of this PR is to break src/util.cpp and src/util.h into the following files:

For Across Platform:

• src/util/platform_common.h
• src/util/platform_common.cpp

For Windows:

• src/util/platform_windows.cpp

For Mac & Linux:

• src/util/platform_posix.cpp

Issue Reference: #12697

Discussion

1. How should we handle "duplicate codes"? For example: this & this
2. Certain things seems to be "very specific to a certain posix platform". How do we want to handle this? For example: this & this

Concept ACK

FWIW, these are places where we use OS specific code after this PR:

$ git checkout bitcoin-12697
$ git grep -E "#if.*(__APPLE__|__DragonFly__|__FreeBSD__|__linux__|MAC_OSX|_MSC_VER|\
__NetBSD__|__OpenBSD__|WIN32|_WIN32_IE|WIN32_LEAN_AND_MEAN|_WIN32_WINNT|_WIN64)" -- \
"*.cpp" "*.h" | grep -vE '(src/secp256k1/|src/leveldb/)' | cut -f1 -d: | sort | uniq -c
      2 src/bitcoind.cpp
      1 src/compat/byteswap.h
      9 src/compat.h
      1 src/dbwrapper.cpp
      1 src/httpserver.cpp
     11 src/init.cpp
      7 src/netbase.cpp
      5 src/net.cpp
      1 src/net.h
      1 src/protocol.cpp
      4 src/qt/guiutil.cpp
      1 src/qt/test/test_main.cpp
      1 src/qt/utilitydialog.cpp
      1 src/qt/winshutdownmonitor.h
      8 src/random.cpp
      1 src/script/bitcoinconsensus.h
      2 src/support/cleanse.cpp
      5 src/support/lockedpool.cpp
      1 src/test/test_bitcoin_fuzzy.cpp
      4 src/test/util_tests.cpp
      1 src/tinyformat.h
      1 src/torcontrol.cpp
     14 src/util/platform_common.cpp
      2 src/util/platform_common.h
      8 src/util/platform_posix.cpp
      7 src/util/platform_windows.cpp
      2 src/utilstrencodings.cpp
      3 src/utiltime.cpp
      1 src/wallet/db.cpp

And for QT OS dependence:

$ git checkout bitcoin-12697
$ git grep -E '#if.*Q_OS_' -- "*.cpp" "*.h" | cut -f1 -d: | sort | uniq -c
      3 src/qt/bitcoin.cpp
     12 src/qt/bitcoingui.cpp
      1 src/qt/bitcoingui.h
      2 src/qt/guiutil.cpp
      1 src/qt/guiutil.h
      4 src/qt/notificator.cpp
      1 src/qt/notificator.h
      2 src/qt/optionsdialog.cpp
      1 src/qt/rpcconsole.cpp
      1 src/qt/winshutdownmonitor.cpp

Your both points...

1. How should we handle "duplicate codes"? For example: this & this
2. Certain things seems to be "very specific to a certain posix platform". How do we want to handle this? For example: this & this

... make me think it's not worth the effort. Maybe it's better to keep it as it is since a clear separation without duplicating code seems not to be possible.

Closing the PR per #13209 (comment)