ebuild: fetch: Factor out _get_fetch_resume_size

ebuild: fetch: Factor out _get_fetch_resume_size

The current fetch() function is quite long, which makes it hard to
know what I can change without adverse side effects.  By pulling this
logic out of the main function, we get clearer logic in fetch() and
more explicit input for the config extraction.

Following a suggestion by Tom Wijsman, I put the setting name in a new
'key' variable to cut down on the PORTAGE_FETCH_RESUME_MIN_SIZE noise.