[BUG] the "source" keyword must first clear optional settings

[BUG] the "source" keyword must first clear optional settings

Problem reported by John Lauro. When "source ... usesrc ..." is
set in the defaults section, it is not possible anymore to remove
the "usesrc" part when declaring a more precise "source" in a
backend. The only workaround was to declare it by server.

We need to clear optional settings when declaring a new "source".
(cherry picked from commit 368480cf4570a0d6448741c704aebd53ac467aa9)