Don't show duplicate lines (per default) such as
sr: resource: Failed to locate 'saleae-logic16-fx2.fw'.
sr: resource: Failed to open resource 'saleae-logic16-fx2.fw'.
The first one is now an sr_dbg() instead of sr_err().
Also, mention that a higher loglevel will give more information as to
where the backend is looking for resources / firmware files.
This fixes bug #806.
The resource API provides a generic means for accessing resources
that are bundled with sigrok, such as device firmware files. Since
the manner of resource bundling is platform-dependent, users of
libsigrok may override the functions used to open, close and read
a resource. The default implementation accesses resources as files
located in one of the XDG data directories or a directory defined
at compile time.
Uwe Hermann
Daniel Elstner