userspace/tools: improve wrs_vlans

--Change parameter --pumask to --puntag; "mask" part is misleading, since full
  mask is not supported.
--use "pmode" rather "pmode"
--improve help message
--support -1 for pprio to disable fix priority
--support -1 for rprio to disable fix priority
--change VLANS_PORTXX_UMASK_ALL and VLANS_PORTXX_UMASK_NONE into
  VLANS_PORTXX_UNTAG_ALL and VLANS_PORTXX_UNTAG_NONE

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