Change handling of PEEK-CHAR and UNREAD-CHAR so that it's done in the

Change handling of PEEK-CHAR and UNREAD-CHAR so that it's done in the
generic I/O port operations.  This is easy to handle by simple hacking
of the byte-buffer indexes, and provides better semantics when the
port coding is changed on the fly.

This breaks transcripting, which must also be migrated to the generic
operations.

Add PEEK-CHAR and UNREAD-CHAR operations to ports that don't inherit
the generic operations.