Bug #247370 - Use a private PORTAGE_TMPDIR for --fetchonly mode in order

Bug #247370 - Use a private PORTAGE_TMPDIR for --fetchonly mode in order
do avoid locking the normal build dir location (similar to bug #245231).
The EbuildFetchPretend class is renamed to EbuildFetchonly and it's used
for --fetchonly mode regardless of whether --pretend is enabled. Also,
fix stdout/stderr flushing in JobStatusDisplay.displayMessage() and
portage.spawn() to ensure output always occurs in the correct order when
displaying pkg_nofetch() output in --fetchonly mode.

svn path=/main/trunk/; revision=12006