P_OPENCLOSE mode flag is *not* specified, do not malloc()/read() the entire
file and then display. Instead, print one line at a time (so long as that
line is <= 1MB in length), calling putmsg() for each line.

This should allow the viewing of massive text files in SBBS again without
alloc/swap issues.

I left the calls to utf8_normalize_str() in here, but I'm not so sure about
them now. putmsg() will convert UTF-8 to CP437 through print_utf8_as_cp437(),
and that only lacks a couple of conversions that utf8_normalize_str() does
(e.g. ellipsis to "..."), so that might be a good candidate to remove in
the future.