Remove srv_win_file_flush_method

- Rename srv_unix_file_flush_method to srv_file_flush_method, and
  rename constants to remove UNIX from them, i.e SRV_UNIX_FSYNC=>SRV_FSYNC

- Add SRV_ALL_O_DIRECT_FSYNC corresponding to current Windows default
(no buffering for either log or data, flush on both log and data)

- change os_file_open on Windows to behave identically to Unix wrt
O_DIRECT and O_DSYNC settings. map O_DIRECT to FILE_FLAG_NO_BUFFERING and
O_DSYNC to FILE_FLAG_WRITE_THROUGH

- remove various #ifdef _WIN32