R4RS 6.10.2  (char-ready? input-port)  ==>  boolean
             (char-ready?)             ==>  boolean

Returns #T if a character is ready on the INPUT-PORT and returns
#F otherwise. If CHAR-READY? returns #T then the next READ-CHAR
operation on the given port is guaranteed not to hang. If the
port is at end of file then CHAR-READY? returns #T. INPUT-PORT may
be omitted, in which case it defaults to the value returned by
CURRENT-INPUT-PORT.

Rationale: CHAR-READY? exists to make it possible for a program to
accept characters from interactive ports without getting stuck
waiting for input. Any input editors associated with such ports
must ensure that characters whose existence has been asserted by
CHAR-READY? cannot be rubbed out. If CHAR-READY? were to return #F
at end of file, a port at end of file would be indistinguishable
from an interactive port that has no ready characters.

NOTE: In S9fES this procedure is part of the SYS-UNIX extension
and not of the core system.
