Reverting "Write subprocess output to a temporary file before reading it into memory. Should get rid of broken pipe error on FreeBSD (hopefully)."
Reverting "Make sure subprocess communication has 4k of buffer."
Revert "Make sure subprocess communication has 4k of buffer."

This reverts commits d8341e3a332e42325b99a59fa2a7ae3888677d32 and
ae625819b5952ef9dec7ecd1ae4dbc6e9d74003d.

Errors on FreeBSD turned out to be unrelated to the stdout pipe.
Add workaround for bad exit codes on BSD.
When trying to get data for non-existing series, print a warning instead of
crashing (allows plotting with missing data series).
Update CHANGES.