isdn: fix a few Kconfig imperfections

1. Rewrite the outdated help texts for config options ISDN and ISDN_CAPI.
2. The MISDN config option appeared between ISDN_I4L and the I4L hardware
   driver options; move it to a less irritating place.
3. HYSDN is not in fact an I4L driver, and needn't depend on ISDN_I4L, so
   move it from the I4L section to the general section.
4. ISDN_HDLC is now also used by drivers outside I4L.  Move it from the
   I4L section to the general section, too.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>