lib: add subtraction helper

it also remove an useless printf

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
tools: new version of fmc-tdc-read

I wrote a new version of the fmc-tdc-read in order to avoid to
use the functions from test-common. In this way, the new
fmc-tdc-tstamp is also a complete self-contained example.

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>