linknx: convert to procd

Removed options UCI parameter. It's not terribly useful. Most of the
parameters can be replaced with procd functionality. procd also demands
processes to run in the foreground.

Removed -fno-builtin. It seems to be a legacy option.

Ran init script through shellcheck.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

net/linknx/files/linknx.config

@@ -1,8 +1,3 @@
-config daemon args
-        # daemon is started as 'linknx --config=$conf $options'
-        # use 'linknx --help' to get all possible options'
-        #
-        # typical example
+config args
         option conf '/etc/linknx.xml'
-        option options '-w --daemon=/tmp/linknx/linknx.log --pid-file=/var/run/linknx.pid'