resolve: always read /etc/resolv.conf on reload systemd-resolved

resolve: always read /etc/resolv.conf on reload systemd-resolved

When systemd-resolved is used in resolv.conf foreign mode, reloading the
systemd-resolved daemon does not read resolv.conf when not touched since
the last time resolved read the file. Since the DNS servers list is
flushed during a reload, resolved forgets about the servers
in /etc/resolv.conf

When reloading the daemon it is expected that all configuration is
reloaded regardless of the disk timestamps.