USEVC option in libc resolv [tcp DNS], how?

[grarpamp]

How to set and use the RES_USEVC / usevc option as mentioned in:

/usr/src/lib/libc/net/resolver.3
/usr/src/include/resolv.h
/usr/src/lib/libc/net/res_debug.c
/usr/src/share/man/man5/resolver.5  # options statement / env_var

The usevc option does not work when used in place of the debug
option below. The debug option works.

env - RES_OPTIONS=debug host freebsd.org
 or
resolv.conf: options debug
env - host freebsd.org

I find usevc in these:
/usr/src/include/resolv.h:#define RES_USEVC     0x00000008      /* use
virtual circuit */
/usr/src/lib/libc/net/gethostbydns.c:           _res.options |=
RES_STAYOPEN | RES_USEVC;
/usr/src/lib/libc/net/gethostbydns.c:   _res.options &= ~(RES_STAYOPEN
| RES_USEVC);
/usr/src/lib/libc/net/getnetbydns.c:            _res.options |=
RES_STAYOPEN | RES_USEVC;
/usr/src/lib/libc/net/getnetbydns.c:    _res.options &= ~(RES_STAYOPEN
| RES_USEVC);
/usr/src/lib/libc/net/name6.c:          _res.options |= RES_STAYOPEN |
RES_USEVC;
/usr/src/lib/libc/net/name6.c:  _res.options &= ~(RES_STAYOPEN | RES_USEVC);
/usr/src/lib/libc/net/res_debug.c:      case RES_USEVC:         return "usevc";
/usr/src/lib/libc/net/res_send.c:       v_circuit = (_res.options &
RES_USEVC) || buflen > PACKETSZ;
/usr/src/lib/libc/net/res_send.c:               if ((v_circuit &&
(!(_res.options & RES_USEVC) || ns != 0)) ||
/usr/src/lib/libc/net/resolver.3:.It Dv RES_USEVC
/usr/src/lib/libc/net/resolver.3:.Dv RES_USEVC

My guess is that it is missing from res_init.c? And that res_init.c
may need to be brought fully up to date with whatever the current
option set is nowadays?

res_init.c:545 /* XXX - print a warning here? */
 On an invalid option, yes please, inform userland :)

This note refers to RELENG_4. It may also apply to RELENG_7 and HEAD.