userspace/tools: improve checking wrs_vlans parameters

--Error when parameters are passed in a form like "1a" (digit followed by a
  not digit)
--For access mode, error when VID is not set.
--Error when parameters in dot-config are "", if are completely empty, then
  libwr will error.

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>