Small changes to bridge-utils to address some minor issues.

1. The default branch is main not master

2. Fixed some compiler warnings because Gcc 10 and Clang now
   do checks for string overflow.

3. Made a backup repository mirror at github.

4. Fixed version string printed

This is not a required update, none of these are important
or critical to users.

Do not intend to do any new features or serious fixes to
bridge-utils; all users should convert to iproute2/bridge command.