test: check for sshd first
test: disable privilege separation
test: fix Buffer buffering and stream handling
SFTPv3: fix undefined error in readdir
SFTPv3: strip out '.' and '..' from readdir list

This matches behavior of node's fs.readdir()
SFTPv3: fix copy-paste error
SFTPv3: truncate decimal when making timestamp
SFTPv3: always update file position in streams
SFTPv3: guard end usage in ReadStream
test: add some sftp tests