make the dest field with -b optional

Often times with -b, you want to bind mount the source into the same
location (like when creating chroots), so you have a lot of flags like:
  -b /var,/var
  -b /dev/log,/dev/log

Make the destination default to the source so now these are equivalent:
  -b /var
  -b /dev/log

Bug: None
Test: `sudo ./minijail0 --profile minimalistic-mountns -b /dev/log /usr/bin/logger foo` works
Change-Id: I4dcf22c1d58e68535c3542d10f80974561d680c5
2 files changed