MEDIUM: proxy: implement named default-server

MEDIUM: proxy: implement named default-server

Implement the support for named default-server. In a single backend, it
is now possible to define different default-server instances identified
by a name. Along to them, it's still possible to use the anonymous
default-server.

To define a named default-server, a new "name" argument can be used just
after the "default-server" token. It is positional to ensure extra
server settings are always set after it. This is necessary to continue
to allow to define a default-server on multiple lines, concatenating the
settings with the previous line.

Named default-server are stored in a new dedicated compact tree
<defsrv_by_name> in proxy struct.

Name must not conflict with server instances already defined in the same
backend. This is necessary for the future implementation of name
addressing on both servers and default-servers without having to specify
the lookup list.