[RFC][PATCH] Add envvar IO_BUFMODE to forcibly set buffering mode

[Sunyeop Lee]

Thanks for comment. I'll write a documentation and tests. I understand
it breaks standard semantics, but does it need to be disabled for
setuid programs even if it's secure? Anyone already can force "line
buffering mode" using pty even on setuid programs. I'm asking if it's
possible to allow only forced "line buffering mode" for setuid
programs instead of disabling it.