mp_msg: print messages to stdout, statusline to stderr

mp_msg: print messages to stdout, statusline to stderr

Previously, mp_msg() printed text to either stderr or stdout depending
on verbosity level (errors/warnings to stderr, rest to stdout). This
split had no significant benefits that I know of, and made capturing
log output harder as you needed to capture both stdout and stderr.
Print all log output to stdout, statusline to stderr. Treat other
updated terminal output the same as statusline.