MINOR: httpclient: handle unix and other socket types in dst

MINOR: httpclient: handle unix and other socket types in dst

httpclient_set_dst() allows one to set the destination address instead
of using the one in the URL or resolving one from the host.
This function also support other types of socket like sockpair@, unix@,
anything that could be used on a server line.

In order to still support this behavior, the address must be set on the
backend in this particular case because the frontend connection does not
support anything other than ipv4 or ipv6.