Fix Icom IC-756 filter handling.

The IC-756 only has two filters per mode unlike the later IC-756
variants that have three. The code was sending invalid commands
due to trying to select the third filter for certain bandwidths.

Also the default filter bandwidths were incorrect for this model.

Signed-off-by: Nate Bargmann <n0nb@n0nb.us>
Force libhamlib to be relinked when a backend is updated

Add the rig and rotor backends to the list of dependencies for
libhamlib.la so that updating a given backend will force libhamlib.la to
be relinked as well.  Thanks to Bill Sommerville, G4WJS, for reporting
this.

Removed obsolete references and variables related to LTDL.