Unify node handling functions:

Unify node handling functions:
* node_alloc() for mdoc and man_node_alloc() -> roff_node_alloc()
* node_append() for mdoc and man_node_append() -> roff_node_append()
* mdoc_head_alloc() and man_head_alloc() -> roff_head_alloc()
* mdoc_body_alloc() and man_body_alloc() -> roff_body_alloc()
* mdoc_node_unlink() and man_node_unlink() -> roff_node_unlink()
* mdoc_node_free() and man_node_free() -> roff_node_free()
* mdoc_node_delete() and man_node_delete() -> roff_node_delete()
Minus 130 lines of code, no functional change.