emerge-webrsync updates

Two small changes for emerge-webrsync.

The first one is that portage_fetch_t must be able to execute
portage_fetch_tmp_t. This is because portage_fetch_t calls portageq,
which (as a Python app) creates a temporary executable file to parse.

The second change allows portage_fetch_t to read user content (when an
overlay is in /home) optionally through the portage_read_user_content
boolean.