1) The yp_bind/yp_unbind and internal _yp_dobind/_yp_unbind sequences shared

1) The yp_bind/yp_unbind and internal _yp_dobind/_yp_unbind sequences shared
dom_binding structs between threads, which is unsafe -- example, dom_vers
signalled retry events, and structs+socket would get deallocated in _yp_unbind.
Change all yp_first (and similar) functions to understand that _yp_dobind now
provides a private dom_binding and socket, which must be released using
_yp_unbind.  Use similar methods in the one-step yp_all function.
2) domainname caching in get* is not neccessary now that the domainname cannot
change relative to ypconnect(2)'s decisions.
Many fields in dom_binding struct become unused, so delete them.
ok jmatthew, also tested by miod