[PATCH] Target stderr not displayed thru MI

[Jim Blandy]

Adding a new MI output stream with its own magic character will break
existing MI clients.  I don't think that's worthwhile, especially
given that, when a program is talking to a TTY, you can't distinguish
the two either.

If, at some point in the future, the MI protocol does get extended to
distinguish the two streams, the code will be ready for it, since we
already distinguish gdb_stdtarg and gdb_stdtargerr internally.  This
change doesn't make future improvements difficult.