journal-remote: do not attempt to read from µhttpd connections

This chunk got lost in one of the rebases :(
microhttpd-util: avoid double free on error

It seems that resources are properly deallocated by MHD_destroy_response,
even if enqueuing the request fails.

Also replace a trivial printf with alloca and fixup log message
(it'll now be something like "Connection from CN=some.host.name",
which seems clear enough.)