tool_cb_wrt: fix invalid unicode for windows console

tool_cb_wrt: fix invalid unicode for windows console

- Suppress an incomplete UTF-8 sequence at the end of the buffer.

- Attempt to reconstruct incomplete UTF-8 sequence from prior call(s)
  in current call.

Prior to this change, in Windows console UTF-8 sequences split between
two or more calls to the write callback would cause invalid "replacement
characters" U+FFFD to be printed instead of the actual Unicode
character. This is because in Windows only UTF-16 encoded characters are
printed to the console, therefore we convert the UTF-8 contents to
UTF-16, which cannot be done with partial UTF-8 sequences.

Reported-by: Maksim Arhipov
Fixes https://github.com/curl/curl/issues/9841
Closes https://github.com/curl/curl/pull/10890