'safe' non-interactive download

[ehuggett]

The downloader may not know the name of a file in advance, but may still wish to download and save it to the suggested filename regardless.

• Sanity checking of the suggested filename is currently performed by the user (there is no sanity checking of the filename)
• How should send-cli handle filename conflicts? (interactively & non-interactively)
• presumably anyone using sendclient as a without send-cli may also find this useful

[dsnezhkov]

+1 for a non-interactive use option, if used as a library. Thank you.