ungetc - push character back into input stream
int ungetc (c, stream)
pushes the character
back on an input stream. That character will be returned by the next
call on that stream.
One character of pushback is guaranteed provided something has been
read from the stream and the stream is actually buffered. Attempts to
push EOF are rejected.
erases all memory of pushed back characters.
returns EOF if it can't push a character back.