BUG/MINOR: server: 'source' interface ignored from 'default-server' directive

BUG/MINOR: server: 'source' interface ignored from 'default-server' directive

Sebastien Gross reported that 'interface' keyword ('source' subargument)
is silently ignored when used from 'default-server' directive despite the
documentation implicitly stating that the keyword should be supported
there.

When support for 'source' keyword was added to 'default-server' directive
in dba97077 ("MINOR: server: Make 'default-server' support 'source'
keyword."), we properly duplicated the conn iface_name from the default-
server but we forgot to copy the conn iface_len which must be set as well
since it is used as setsockopt()'s 'optlen' argument in
tcp_connect_server().

It should be backported to all stable versions.