sys/shell: ncput: add option to read from stdin

[benpicco]

Contribution description

ncput is a useful command line tool to interact with a CoAP server.
In addition to uploading files, also add an option to put data directly from the command line in the same fashion was we can tell ncget to output to the command line instead of to a file.

Testing procedure

• in one terminal, start examples/nanocoap_server
• in another terminal, start tests/nanocoap_cli:

> ncget coap://[fe80::581a:98ff:fe23:2d6c]/.well-known/core -
</.well-known/core>,</echo/>,</riot/board>,</riot/value>,</riot/ver>,</sha256>

> ncget coap://[fe80::581a:98ff:fe23:2d6c]/riot/value -
0

> ncput - coap://[fe80::581a:98ff:fe23:2d6c]/riot/value 23
Saved to coap://[fe80::581a:98ff:fe23:2d6c]/riot/value

> ncget coap://[fe80::581a:98ff:fe23:2d6c]/riot/value -
23

Issues/PRs references

[chrysn]

Looks good to me, tested with aiocoap-fileserver.

The CLI of using - to refer to a last argument is a bit weird (I'd have expected it to point to stdin), but it's still straightforward enough, and the error message is helpful.