[NET] AX.25 Kconfig and docs updates and fixes

 o The AX.25 Howto is unmaintained since several years.  I've replaced it
   with a wiki at http://www.linux-ax25.org which provides more uptodate
   information.
 o Change default for AX25_DAMA_SLAVE to Y.  AX25_DAMA_SLAVE only compiles
   in support for DAMA but doesn't activate it.  I hope this gets Linux
   distributions to ship their AX.25 kernels with AX25_DAMA_SLAVE enabled.
   The price for this would be very small.
 o Delete historic changelog from comments, that's what SCM systems are
   meant to do.
 o ---help--- in Kconfig looks so yellingly eye insulting.  Use just help.
 o Rewrite the commented out piece of old Linux 2.4 configuration language
   to Kconfig for consistency.
 o Fixup dependencies.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>