Instead of waiting, GetC always return immediately, with Z indicating if
something was fetched. The "wait" loop is implemented by the called (and
in the new `blkGetCW`).
This simplifies out-of-bounds verifications for storage blockdevs.
They serve a different purpose. The goal of the ACIA buffer is to ensure
that we don't miss an input. The goal of the shell buffer is to wait
until the user presses return.
The ACIA buffer has been moved to shell and replaced with a circular
buffer, a more appropriate data structure for this kind of purpose.
Also, introduce `aciaGetC`.
