journal: make sure to set MHD_USE_PIPE_FOR_SHUTDOWN for libmicrohttpd servers

This makes sure libmicrohttpd won't call shutdown() on our listening
sockets, which make sure socket activation and re-activation will work
cleanly.

See:

https://github.com/systemd/systemd/pull/1286
https://lists.gnu.org/archive/html/libmicrohttpd/2015-09/msg00014.html

Fixes #1286