tweak a log string and a comment
add initial curl wrapper, but known currently not to be picked up correctly by installer
create top-level directories for cache and logfile as necessary
The curl wrapper is in principle already working; tweak some comments, and add a simple test script that proves the point.
What appears to be breaking it inside esg-node is that esg-node prepends to PATH after building curl (contrast with wget, that is a prerequisite and not built at run time). Will fix this in the next commit...
Prevent esg-node from putting the newly built curl into the path. This still allows curl to be built, for sake of any other scripts that might require it (to be run later not via esg-node), but esg-node itself and any scripts which it calls will use the curl wrapper instead (which uses wget or the cache).