New build system for the external tree

In the past it was a problem to compile mISDN for newer kernel versions
which already have mISDN, you need to delete files in the kernel or
patch the kernel with the new version.
A second issue was, that some files in the standalone tree (Makefile and
some header files) were different to the in kernel version to allow proper
standalone builds. This caused higher maintenance effort to generate patches
for the kernel and for back porting kernel patches.
To solve this, I started to use autoconf/automake and generate a copy of the
driver tree for standalone compiles. The files are processed with sed to
make sure that here are no naming conflicts with the kernel.

1. The mISDN include files from include/linux are renamed (*.h --> *_s.h)
2. All CONFIG_MISDN* defines are changed to _MCONFIG_MISDN

These actions make it possible to compile mISDN standalone without
any influence from the in kernel mISDN version.

Steps to compile:

1. run ./configure
2. Copy mISDN.cfg.default to standalone/mISDN.cfg and edit it for you needs
   It contain a list with all CONFIG_MISDN* defines, one per line
   If you want to unset one feature, please set a '#' before it (do not
   remove the line!)
3. make modules
4. make modules-install