SFTP: add more stat functions

also don't use stat cache for SFTP/Stream.php
SFTP: fix syntax error
SFTP: NULL -> null
SSH2: make it so callback functions can make exec() return early
SSH2: 0x7FFFFFFF -> $this->window_size in one more place
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
Merge pull request #349 from terrafrost/ssh2-exec-callback-termination

SSH2: make it so callback functions can make exec() return early

* terrafrost/ssh2-exec-callback-termination:
  SSH2: 0x7FFFFFFF -> $this->window_size in one more place
  SSH2: make it so callback functions can make exec() return early
SFTP/Stream: use $scheme instead of 'sftp'

This makes it easier for someone to change the scheme (which can be
done by changng the stream_wrapper_register call at the bottom
of the file)
SFTP/Stream: add explanation as to why nlist() is used here
SFTP/Stream: fix capatilization
Merge pull request #352 from terrafrost/sftp-stream-changes

SFTP Stream changes

* terrafrost/sftp-stream-changes:
  SFTP/Stream: fix capatilization
  SFTP/Stream: add explanation as to why nlist() is used here
  SFTP/Stream: use $scheme instead of 'sftp'