MINOR: server: define _srv_parse_from() for server "from" keyword

MINOR: server: define _srv_parse_from() for server "from" keyword

Prepare the support for a new server keyword "from".

This keyword has special constraints : it is a positional one as it can
only be specified once, after the server address and before the other
parameters.

The purpose of this keyword will be to define server settings
inheritance outside of the default-server of the current backend. It
will also be useful for dynamic servers which currently do not inherit
from a default-server.