Avoid delay waiting for key in WIN_DYN mode
When running Windows 11 in a VirtualBox VM, Sleep(1) appears to take
much more than 1ms to return. This causes large delays drawing menus
using WIN_DYN (as SyncTERM does).
If WIN_DYN is set, just use kbhit() with no delay. Leave the delay
there for !WIN_DYN though.
This should generally speed up SyncTERM menu navigation as there
was a 50ms delay for each menu on the screen.
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