Add openssh sftp-server path detection for windows

[afbjorklund]

Unfortunately LookPath doesn't handle DOS extensions.

But it was already confused by the / location anyway.

i.e. LookPath("/usr/lib/openssh/sftp-server") fails

Even though the Windows program works fine to execute.

---

Testing on Ubuntu:

$ export WINEPATH="C:\\Program Files\\qemu;C:\\Program Files\\Git\\usr\\bin"
$ wine cygpath -w /usr/lib/ssh/sftp-server | cat
Cygwin WARNING:
  Couldn't compute FAST_CWD pointer.  This typically occurs if you're using
  an older Cygwin version on a newer Windows.  Please update to the latest
  available Cygwin version from https://cygwin.com/.  If the problem persists,
  please see https://cygwin.com/problems.html

C:\Program Files\Git\usr\lib\ssh\sftp-server.exe

Spaces and carriage returns, oh my.

I had first installed Git for Windows

[AkihiroSuda]

Thanks