.bashrc.d/20nobeep: Fallback to -blength if --blength doesn't work

.bashrc.d/20nobeep: Fallback to -blength if --blength doesn't work

I'd bumped from -blength to --blength in abee64c4 (.bashrc.d/20nobeep:
Change '-blength' -> '--blength', 2015-03-19), expecting to never see
util-linux < 2.25 or so again.  But I've recently been exposed to a
RHEL 7.5 system, which has:

  $ setterm -version
  setterm from util-linux 2.23.2
  $ setterm --help 2>&1 | grep blength
   -blength <0-2000>

so this commit adds code to attempt the single-dash form if the
double-dash form fails.