Make all temp dirs under $PORTAGE_TMPDIR/portage.

Make all temp dirs under $PORTAGE_TMPDIR/portage.

Before, some temporary directories would be created directly in
$PORTAGE_TMPDIR. Now, all are subdirectories of $PORTAGE_TMPDIR/portage
since it's common for people to assume that this is the case anyway.
With the default PORTAGE_TMPDIR setting of /var/tmp, this allows
/var/tmp to be mounted with the "noexec" option, as long as the
/var/tmp/portage subdirectory is a separate mount (people have already
tended to assume that they can do this, so we're making it a reality
in order to avoid any more bug reports). This will fix bug #346899.