isdn/capi: fix (middleware) device nodes

Since v2.4 the capi driver used the following device nodes if
"middleware" support was enabled:
    /dev/capi20
    /dev/capi/0
    /dev/capi/1
    [...]

/dev/capi20 is a character device node. /dev/capi/0 (and up) are tty
device nodes (with a different major).

This device node (naming) scheme is not documented anywhere, as far as I
know. It was originally provided by the capifs pseudo filesystem (before
udev became available). It is required for example by the pppd
capiplugin. It was supported until a few years ago. But a number of
developments broke it:
- v2.6.6 (May 2004) renamed /dev/capi20 to /dev/capi and removed the
  "/" from the name of capi's tty driver. The explanation of the patch
  that did this included two examples of udev rules "to restore the old
  namespace";
- either udev 154 (May 2010) or udev 179 (January 2012) stopped
  allowing to rename device nodes, and thus the ability to have
  /dev/capi20 appear instead of /dev/capi and /dev/capi/0 (and up)
  instead of /dev/capi0 (and up);
- v3.0 (July 2011) also removed capifs. That disabled another method to
  create the /dev/capi/0 (and up) device nodes.

So now users need to manually tweak their setup (eg, create /dev/capi/
and fill that with symlinks) to get things working. This is all rather
hacky and only discoverable by searching the web. Fix all this by
renaming /dev/capi back to /dev/capi20, and by setting the name of the
"capi_nc" tty driver to "capi!" so the tty device nodes appear as
/dev/capi/0 (and up).

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>