line-buffer stdout to be nice to debug engines

[tlyu]

Some IDE debug engines start up debug servers in pipes and look for
status messages to indicate the debug server is finished starting
up. The startup messages could be stuck in the stdio buffer,
causing the debug engine to time out.

[thierer]

I can confirm this is useful to get debugging via avarice to work in vscode.

The patch doesn't work for me (linux, glibc 2.40) in its current form, though; I have to move the setvbuf() call all the way up before the first statusOut() in main(). (It does work in its current position with _IONBF instead of _IOLBF).