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.