netgraph/ng_bridge: Make simple internal functions read-only

[donner]

Summary

Switch very simple internal functions to explicit handle const data.

Depends on D28123

[donner]

revert the idea of specialized "const pointers"

[donner]

It's far that simple. I missed the difference between

• const node_p: a constant pointer to a modifiable node.
• struct ng_node const * : a modifiable pointer to a constant node.

Because I need immutable data structures, the latter is required.
But those structures and helper functions are defined in the central netgraph.h and ng_base.c.
Hence I have to annotate the base system with const, and also every helper function, which does not modify the data.

I think it's worth the effort, because it will ease the way write multithreaded netgraph modules and fix several still hidden race conditions.

[donner]

• Fix some formatting, line up comments.
• Annotate helper functions to handle immutable objects.

[donner]

Rebase due to major changes

[kp]

LGTM