SFTP: add more stat functions

also don't use stat cache for SFTP/Stream.php
SFTP: fix syntax error
SFTP: NULL -> null
Merge pull request #348 from terrafrost/more-sftp-changes

More sftp changes

* terrafrost/more-sftp-changes:
  SFTP: NULL -> null
  SFTP: fix syntax error
  SFTP: add more stat functions