Add a fallback case to winsock_error_string() which makes it call

FormatMessage to get the OS's text for any error not in our own
translation table. Should eliminate the frustrating 'unknown error'.

(I haven't chosen to use FormatMessage unconditionally, because it
comes out with enormous messages along the lines of "No connection
could be made because the target machine actively refused it" in place
of "Connection refused" and I'm Unixy enough to prefer the latter.
Also, on older Windowses, Winsock error codes are in a separate API
segment and don't work with FormatMessage anyway.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@9704 cda61777-01e9-0310-a592-d414129be87e