Same idea as 03677889f0.
No functional change intended. The type of the iterator is generally changed to
be 'const char*' instead of 'char*'. Despite the type commonly used, modifying
the string was not allowed.
I adjusted the naming of some short variables for clarity and reduced the scope
of some variable declarations in code that was being touched anyway.
People often assigns the MAC address of the enslaved interface to e.g.
bridge interface. So, the local assignment bit should not be adjusted.
Fixes#21649.
Zbigniew Jędrzejewski-Szmek
Yu Watanabe