cmd: when running --password-command allow use of stdin

Bind rclone standard input to password command's standard input. This
allows to provide password from a pipe and collect it using cat.

The typical use case is when rclone is on a remote server with an
encrypted configuration. This solved the environment variable
issue (#3368) and the password storage on remote host.

Now the following chain is allowed:

    echo 'secret' | ssh host.example.com \
       sudo -u rclone \
       rclone --config /path/to/rclone.conf \
       --password-command 'cat' ls remote:

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>

Co-authored-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>

fs/config/config.go

@@ -282,6 +282,7 @@ func loadConfigFile() (*goconfig.ConfigFile, error) {
 
 			cmd.Stdout = &stdout
 			cmd.Stderr = &stderr
+			cmd.Stdin = os.Stdin
 
 			if err := cmd.Run(); err != nil {
 				// One does not always get the stderr returned in the wrapped error.