Use regular kernel file descriptors for all kinds of connection
handles, referring to regular file objects:

- connection to devnode => filp on chrdev inode
- socket => filp on anon inode

User and kernel-originated connection handles are no more
differenciated.

The former procfs interface for RTDM is now entirely gone, as all file
descriptors are now visible from /proc/<pid>/fd.

Obtaining the list of all named devices can be done this way:

$ grep named /sys/class/rtdm/*/flags

Similarly, the list of all protocol devices can be obtained this way:

$ grep protocol /sys/class/rtdm/*/flags

Force closing a stuck file descriptor (formerly done by writing to
rtdm/open_fildes) makes no sense anymore. Either the owner is a
userland process and all file handles it holds will be automatically
dropped at exit, or it is a kernel driver, in which case the driver
direly needs fixing.