Drop libftdi 0.x support, require libftdi 1.x.

Drop libftdi 0.x support, require libftdi 1.x.

There were some issues when using libftdi 0.x recently that are fixed
when switching to libftdi 1.x (not really worth the effort to investigate).

libftdi 1.x has been around several years now and should be available
in most recent distros and OSes. For the rare cases where it is not,
it's easy enough to get it via backport packages, or build from source,
or sigrok users can just use our binary installers for most OSes.

This also "fixes"/obsoletes bug #959.