Actually cross platform UART code

[ArcaneNibble]

• Windows
• macOS
• BSD?
• Android??

[bvernoux]

Issue to build the actual code with MSYS2 / MINGW64 related to UART.cpp code
-- The C compiler identification is GNU 10.2.0
-- The CXX compiler identification is GNU 10.2.0
...
Error

\home\Ben\scopehal-apps\lib\xptools\UART.cpp:75:9: error: invalid conversion from 'int' to 'FILE_DESCRIPTOR' {aka 'void*'} [-fpermissive]
   75 |  , m_fd(-1)
      |         ^~
      |         |
      |         int

Quick fix was to replace m_fd(-1) by m_fd(INVALID_HANDLE_VALUE) (to be checked if that is correct but it build at least)