MFC r217557: sh: Fix signal messages being sent to the wrong file sometimes.

MFC r217557: sh: Fix signal messages being sent to the wrong file sometimes.

When a foreground job exits on a signal, a message is printed to stdout
about this. The buffer was not flushed after this which could result in the
message being written to the wrong file if the next command was a builtin
and had stdout redirected.

Example:
  sh -c 'kill -9 $$'; : > foo; echo FOO:; cat foo

git-svn-id: svn://svn.freebsd.org/base/stable/8@218337 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f