Correct the layering violation in read_body_to_string. The previous
version called the higher-level archive_read_data and
archive_read_data_skip functions, which screwed up state management of
those functions. This bit of mis-design has existed for a long time,
but became a serious issue with the recent changes to the
archive_read_data APIs, which added more internal state to the
high-level archive_read_data function. Most common symptom was a
failure to correctly read 'L' entries (long filename) from GNU-style
archives, causing the message ": Can't open: No such file or
directory" with an empty filename.

Pointed out by: Numerous port build failures
Thanks to: Kris Kennaway