process: separate stderr from stdout

process: separate stderr from stdout
- Add the option of redirecting stderr to a file. With the old
behaviour, stderr would follow stdout if stdout was to a file, but
stderr went to the host stderr if stdout went to the host stdout.  The
new default maintains stdout and stderr going to the host.  Now the
two can specify different files, but they will share a file descriptor
if the name of the files is the same.
- Add --output and --errout options to se.py to go with --input.