debugger: leave output screen with single key press

Introduce support code to get single key presses from a console. Support
Windows (msvcrt) and POSIX supporting Unix platforms (select, termios).
Fall back to Python's input() routine for Webassembly and Emscripten.

Avoid the accumulation of repeated prompts on platforms that support
single key presses. Only the backwards compatible fallback on minor
platforms involves a prompt, and users can tell when they face that
situation.

This change increases usability, a single key press leaves the output
screen after it was entered by pressing 'o'. Does not require a config
item, platform detection is automatic. The approach uses Python core
libraries, no external dependencies are involved. The implementation
is considered maintainable, lends itself to future extension for more
platforms as the need gets identified.