[MINOR] acl: add srv_is_up() to check that a specific server is up or not

[MINOR] acl: add srv_is_up() to check that a specific server is up or not

This ACL was missing in complex setups where the status of a remote site
has to be considered in switching decisions. Until there, using a server's
status in an ACL required to have a dedicated backend, which is a bit heavy
when multiple servers have to be monitored.