Fixes transfer() issues (#579)

- use subshell to avoid leaking vars to current shell
- use POSIX 'test -t' instead of tty
- have a single place to call curl
- echo the output url to add the carrier return when needed.
- use printf to process \n
- silence curl instead of teeing /dev/null

Co-authored-by: Andrea Spacca <andrea.spacca@gmail.com>