base: switch to cURL multi API for curl fetch and handle signals, https://trac.macports.org/ticket/43280
base: switch to cURL multi API for curl fetch and handle signals, https://trac.macports.org/ticket/43280

This allows users to cancel fetch using SIGINT and SIGTERM (or any other
signal, really) where they'd previously have to wait for the curl operation to
finish completely.
With this change the progress finish callback is always called, even if an
error occurs, which was previously not the case.

git-svn-id: 620571fa9b4bd0cbce9a0cf901e91ef896adbf27@118713 d073be05-634f-4543-b044-5fe20cf6d1d6

danchr/macports-base,macports/macports-base,neverpanic/macports-base,macports/macports-base,danchr/macports-base,neverpanic/macports-base