stty: support [-]drain setting to control waiting for pending Tx

stty: support [-]drain setting to control waiting for pending Tx

Instead of commit v8.24-132-g5171bef which only provides
control to disable this behavior (with -I), provide
the symmetrical "[-]drain" special setting.

* src/stty.c (main): Parse the [-]drain setting instead of -I,
and treat like a global option.
(usage): Adjust accordingly.
* tests/misc/stty.sh: Test "drain" with and without options.
* NEWS: Mention the new feature.