MEDIUM: server: implement from be:

MEDIUM: server: implement from be:

Implement "from be:" syntax. This instructs that the server should reuse
parameters from the default-server already defined in the same proxy
instance.

This behavior is already the current one in the configuration parser,
thus there is no visible change here. The main usage is on "add server"
command, which previously always ignored a default-server instance.

This is implemented by extending _srv_parse_from() to parse "be:"
argument value. If found, the designated server is returned via <from>
output parameter.