Make /output in CALL take string! instead of word!

This makes it in line with R2. In R2, if the string! is not empty, it
seems to insert the output to the string, which I doesn't feel natural.
So in R3, the output is appended to the string.
Take binary! for I/O redirection in CALL

The difference between string! and binary! is that, with string!, the
interpreter will try to do the encoding conversion before/after passing
in/out of the command; with binary!, no such conversion is done in the
interpter, everything will be passed in/out as-is. So if the conversion
is not desirable/correct, use binary! (and do the conversion yourself).