Dav: changed ngx_copy_file() to preserve access and mtime.

Dav: changed ngx_copy_file() to preserve access and mtime.

This fixes wrong permissions and file time after cross-device MOVE
in the DAV module (ticket #1577).  Broken in 8101d9101ed8 (0.8.9) when
cross-device copying was introduced in ngx_ext_rename_file().

With this change, ngx_copy_file() always calls ngx_set_file_time(),
either with the time provided, or with the time from the original file.
This is considered acceptable given that copying the file is costly anyway,
and optimizing cases when we do not need to preserve time will require
interface changes.