MergeProcess: Fix PORTAGE_BACKGROUND/LOG_FILE use

MergeProcess: Fix PORTAGE_BACKGROUND/LOG_FILE use

In this subprocess we don't want PORTAGE_BACKGROUND to suppress
stdout/stderr output since they are pipes. We also don't want to open
PORTAGE_LOG_FILE, since it will already be opened by the parent
process, so we set the PORTAGE_BACKGROUND="subprocess" value for use
in conditional logging code involving PORTAGE_LOG_FILE.