MEDIUM: server: implement "from srv:"

MEDIUM: server: implement "from srv:"

Implement "srv:" notation for the "from" server keyword. This allows to
specify a server or default-server by its name. It may be optionnaly
prefixed by a backend name using a slash separator. If this is not the
case, lookup is performed under the backend where the current newly
created server instance is attached.

This is implemented by extending _srv_parse_from() to support "from
srv:" argument value. An internal function lookup_srv_be_arg() is
defined to parse the argument of the form "[<be>/]<srv>". This is
similar to already existing lookup functions such as cli_find_server(),
however there is some differences which forces to have duplicated code
for the moment.