BUG/MINOR: config: don't propagate process binding for dynamic use_backend

A segfault was reported with the introduction of the propagate_processes()
function. It was caused when a use_backend rule was declared with a dynamic
name, using a log-format string. The backend is not resolved during the
configuration, which lead to the segfault.

The patch prevents the process binding propagation for such dynamic rules, it
should also be backported to 1.5.