[cdc_rsync] [cdc_stream] Switch from scp to sftp (#66)

Use sftp for deploying remote components instead of scp. sftp has the
advantage that it can also create directries, chmod files etc., so
that we can do everything in one call of sftp instead of mixing scp
and ssh calls.

The downside of sftp is that it can't switch to ~ resp. %userprofile%
for the remote side, and we have to assume that sftp starts in the
user's home dir. This is the default and works on my machines!

cdc_rsync and cdc_stream check the CDC_SFTP_COMMAND env var now and
accept --sftp-command flags. If they are not set, the corresponding
scp flag and env var is still used, with scp replaced by sftp. This is
most likely correct as sftp and scp usually reside in the same
directory and share largely identical parameters.

cdc_stream/session.h

@@ -38,8 +38,8 @@ struct SessionTarget {
   std::string user_host;
   // Ssh command to use to connect to the remote target.
   std::string ssh_command;
-  // Scp command to use to copy files to the remote target.
-  std::string scp_command;
+  // Sftp command to use to copy files to the remote target.
+  std::string sftp_command;
   // Directory on the remote target where to mount the streamed directory.
   std::string mount_dir;
 };