replace MSYS2 write() with WriteFile

[dankamongmen]

Now that we're carrying around ti->outhandle, we ought use it with the native WriteFile() call: https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-writefile. In particular, apparently this is necessary to properly use UTF-8 relative to WriteFileConsole(). who knows what Cygwin (or winsock?) write() is doing under the hood.

[dankamongmen]

it does not look like we need any WriteFileW():

As of now, UTF-8 is supported fully on the standard output stream with the WriteConsole and WriteFile methods. Support on the input stream varies depending on the input mode and will continue to improve over time. Notably the default "cooked" modes on input do not fully support UTF-8 yet. The current status of this work can be found at microsoft/terminal#7777 on GitHub. The workaround is to use the algorithmically-translatable UTF-16 for reading input through ReadConsoleW or ReadConsoleInputW until the outstanding issues are resolved.

https://docs.microsoft.com/en-us/windows/console/classic-vs-vt

[dankamongmen]

i really doubt this is what solves #2117, which is what this issue is all about. closing it up.