config: fix one-node dumpconfig, add dm_config_write_one_node

config: fix one-node dumpconfig, add dm_config_write_one_node

A regression introduced in 2.02.89 (11e520256b3005ed813ce83f8770aaab74edef3f)
caused the lvm dumpconfig <node> to print out
the node as well as its subsequent siblings.
The information about "only_one" mode got lost.

Before this patch (just an example node):
  # lvm dumpconfig global/use_lvmetad
  use_lvmetad=1
  thin_check_executable="/usr/sbin/thin_check"
  thin_check_options="-q"
  (...all nodes to the end of the section)

With this patch applied:
   # lvm dumpconfig global/use_lvmetad
   use_lvmetad=1