Made sitedef_node mostly independent of sitedef_node_type.

Changed sitedef_node_type_find_name() function to use parse_node_type_t
instead of enum sitedef_node_type for its second parameter.  Made similar
chage to the sitedef_node_type_info table.

Added needed #includes for sitedef_node_type.h.

Added missing check for result of sitedef_node_type_find_name()
in sitedef_node_alloc_description().