BUG/MINOR: server: Forbid to set fqdn on the CLI if SRV resolution is enabled

BUG/MINOR: server: Forbid to set fqdn on the CLI if SRV resolution is enabled

If a server is configured to rely on a SRV resolution, we must forbid to
change its fqdn on the CLI. Indeed, in this case, the server retrieves its
fqdn from the SRV resolution. If the fqdn is changed via the CLI, this
conflicts with the SRV resolution and leaves the server in an undefined
state. Most of time, the SRV resolution remains enabled with no effect on
the server (no update). Some time the A/AAAA resolution for the new fqdn is
not enabled at all. It depends on the server state and resolver state when
the CLI command is executed.

This patch must be backported as far as 2.0 (maybe to 1.8 too ?) after some
observation period.