Avoid sourcing /etc/bash_completion twice for Debian-based distributions.

Avoid sourcing /etc/bash_completion twice for Debian-based distributions.

On a system running Debian 5.0 (Lenny), running /etc/bash_completion
once sets some variables (e.g. BASH_COMPLETION) and then makes them
read-only.  Running /etc/bash_completion again raises errors when it
tries to reset these read-only variables, and the script bails, taking
down all subsequent .bashrc processing with it.  Work around this
problem by only sourcing /etc/bash_completion if the BASH_COMPLETION
variable is empty.

Later versions of /etc/bash_completion avoid this issue by only
setting the variables if they are empty.

Versions affected:
  Debian 5.0 (Lenny) running /etc/bash_completion 20060301
  Ubuntu 8.04.4      running /etc/bash_completion 20060301
Versions not affected:
  Ubuntu 9.04        running /etc/bash_completion 20080617.5
  Ubuntu 10.04.2     running /etc/bash_completion 1.1 (2009)
  Ubuntu 10.04.3     running /etc/bash_completion 1.1 (2009)

The fix should work with all the versions I've seen.