To avoid linear searches for error codes, autogenerate errmap as
simple array of errno values indexed by Windows error codes.
Restrict to the first 9000 Windows error codes, we don't care for
most of them anyway.
Define errmap in its own file errmap.h to clean up errno.cc.
Change geterrno_from_win_error accordingly.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
