Win32 dirent: improve dirent implementation

Win32 dirent: improve dirent implementation

Improve the dirent implementation by removing the relics that were once
necessary to plug into the now unused MinGW runtime, in preparation for
Unicode file name support.

Move FindFirstFile to opendir, and FindClose to closedir, with the
following implications:
- DIR.dd_name is no longer needed
- chdir(one); opendir(relative); chdir(two); readdir() works as expected
  (i.e. lists one/relative instead of two/relative)
- DIR.dd_handle is a valid handle for the entire lifetime of the DIR struct
- thus, all checks for dd_handle == INVALID_HANDLE_VALUE and dd_handle == 0
  have been removed
- the special case that the directory has been fully read (which was
  previously explicitly tracked with dd_handle == INVALID_HANDLE_VALUE &&
  dd_stat != 0) is now handled implicitly by the FindNextFile error
  handling code (if a client continues to call readdir after receiving
  NULL, FindNextFile will continue to fail with ERROR_NO_MORE_FILES, to
  the same effect)
- extracting dirent data from WIN32_FIND_DATA is needed in two places, so
  moved to its own method
- GetFileAttributes is no longer needed. The same information can be
  obtained from the FindFirstFile error code, which is ERROR_DIRECTORY if
  the name is NOT a directory (-> ENOTDIR), otherwise we can use
  err_win_to_posix (e.g. ERROR_PATH_NOT_FOUND -> ENOENT). The
  ERROR_DIRECTORY case could be fixed in err_win_to_posix, but this
  probably breaks other functionality.

Removes the ERROR_NO_MORE_FILES check after FindFirstFile (this was
fortunately a NOOP (searching for '*' always finds '.' and '..'),
otherwise the subsequent code would have copied data from an uninitialized
buffer).

Changes malloc to git support function xmalloc, so opendir will die() if
out of memory, rather than failing with ENOMEM and letting git work on
incomplete directory listings (error handling in dir.c is quite sparse).

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>