BUG/MINOR: config: Make use_backend and use-server post-parsing less obscur

BUG/MINOR: config: Make use_backend and use-server post-parsing less obscur

During use_backend and use-server post-parsing, if the log-format string used to
specify the backend or the server is just a single string, the log-format string
(a list, internally) is replaced by the string itself. Because the field is an
union, the list is not emptied according to the rules of art. The element, when
released, is not removed from the list. There is no bug, but it is clearly not
obvious and error prone.

This patch should fix #544. The fix for the use_backend post-parsing may be
backported to all stable releases. use-server is static in 2.1 and prior.