DEVFS - Cleanup of dead functions/comments; various fixes

DEVFS - Cleanup of dead functions/comments; various fixes

Cleanup of dead functions and structures and cleanup of C++-style
comments.
Fixes include:
- function devfs_node_is_accessible, which checks if the node is valid
and if it isn't a hidden node.
- calls to devfs_node_is_accessible from most vnops to ensure validity
of the devfs_node they operate on.
- Change of devfs_rule struct to use dynamically allocated memory
instead of using pre-sized buffers. This way we avoid wasting a lot
of memory for each devfs_rule.