resolved: add bus API for configuring per-link DNS settings

This is useful for alternative network management solutions (such as NetworkManager) to push DNS configuration data
into resolved.

The calls will fail should networkd already have taken possesion of a link, so that the bus API is only available if
we don't get the data from networkd.

src/libsystemd/sd-bus/bus-common-errors.c

@@ -78,6 +78,8 @@ BUS_ERROR_MAP_ELF_REGISTER const sd_bus_error_map bus_common_errors[] = {
         SD_BUS_ERROR_MAP(BUS_ERROR_DNSSEC_FAILED,                EHOSTUNREACH),
         SD_BUS_ERROR_MAP(BUS_ERROR_NO_TRUST_ANCHOR,              EHOSTUNREACH),
         SD_BUS_ERROR_MAP(BUS_ERROR_RR_TYPE_UNSUPPORTED,          EOPNOTSUPP),
+        SD_BUS_ERROR_MAP(BUS_ERROR_NO_SUCH_LINK,                 ENXIO),
+        SD_BUS_ERROR_MAP(BUS_ERROR_LINK_BUSY,                    EBUSY),
 
         SD_BUS_ERROR_MAP(BUS_ERROR_NO_SUCH_TRANSFER,             ENXIO),
         SD_BUS_ERROR_MAP(BUS_ERROR_TRANSFER_IN_PROGRESS,         EBUSY),

src/libsystemd/sd-bus/bus-common-errors.h

@@ -77,6 +77,8 @@
 #define BUS_ERROR_DNSSEC_FAILED "org.freedesktop.resolve1.DnssecFailed"
 #define BUS_ERROR_NO_TRUST_ANCHOR "org.freedesktop.resolve1.NoTrustAnchor"
 #define BUS_ERROR_RR_TYPE_UNSUPPORTED "org.freedesktop.resolve1.ResourceRecordTypeUnsupported"
+#define BUS_ERROR_NO_SUCH_LINK "org.freedesktop.resolve1.NoSuchLink"
+#define BUS_ERROR_LINK_BUSY "org.freedesktop.resolve1.LinkBusy"
 #define _BUS_ERROR_DNS "org.freedesktop.resolve1.DnsError."
 
 #define BUS_ERROR_NO_SUCH_TRANSFER "org.freedesktop.import1.NoSuchTransfer"