staprun: fix warning/error/dbug eprintf() races causing garbled output

staprun: fix warning/error/dbug eprintf() races causing garbled output

There are lots of races when printing warnings/errors/dbugs in staprun
because multiple eprintf() calls are used to print a single message and
stderr is not line-buffered. As a result, warnings/errors/dbugs race with
the relayfs reader threads printing to stdout and with other stap scripts
running concurrently in the same PTY. This causes the messages printed to
stderr and stdout to be garbled.

Fix all of this by using a single eprintf() for each warning/error/dbug
message, and by making stderr line-buffered so that we don't need to worry
about differing libc implementations potentially flushing a single message
in chunks rather than flushing the whole message in one go.