Following up on r1619121, refactor the property conflict marker generator code
to allow directly writing markers based on the information in wc.db.

Note that this doesn't update the only caller of this function, to specifically
test the legacy code path.

* subversion/libsvn_wc/props.c
  (prop_conflict_from_skel): Remove forward declaration.
  (append_prop_conflict): Remove. Folded in only caller.

  (prop_conflict_from_skel): Extract description creation into...
  (prop_conflict_new): ... this new function, which is now called by...
  (prop_conflict_from_skel): ... this wrapper with the old name.

  (svn_wc__create_prejfile): Rename argument and allow passing NULL for the
    property data.

* subversion/libsvn_wc/props.h
  (svn_wc__create_prejfile): Rename argument. Document.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619129 13f79535-47bb-0310-9956-ffa450edef68