Override readline's getc function to use Racket's read-byte.

Eliminates the busy-polling behavior observed in Mac OS X when
xrepl is required at the regular repl.

Closes PR 13350.
racket/enter: fix for `enter!' related to submodules

The load handler didn't obey the protocol with respect to a second
argument that is a list starting with #f.

Closes PR 13096