dhcp: make client selection more forgiving

If your distributor forgot to set up the client path for you, this
makes NM look in the right places and allows you to use either client
if it's installed, no matter what NM was configured with.

src/dhcp-manager/nm-dhcp-manager.h

@@ -33,6 +33,18 @@
 #include "nm-dhcp4-config.h"
 #include "nm-hostname-provider.h"
 
+enum {
+    NM_DHCP_MANAGER_ERROR_BAD_CLIENT = 0,
+    NM_DHCP_MANAGER_ERROR_INTERNAL = 1,
+};
+
+#define NM_DHCP_MANAGER_ERROR (nm_dhcp_manager_error_quark ())
+#define NN_TYPE_DHCP_MANAGER_ERROR (nm_dhcp_manager_error_get_type ())
+
+GQuark nm_dhcp_manager_error_quark    (void);
+GType  nm_dhcp_manager_error_get_type (void);
+
+
 #define NM_TYPE_DHCP_MANAGER            (nm_dhcp_manager_get_type ())
 #define NM_DHCP_MANAGER(obj)            (G_TYPE_CHECK_INSTANCE_CAST ((obj), NM_TYPE_DHCP_MANAGER, NMDHCPManager))
 #define NM_DHCP_MANAGER_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST ((klass), NM_TYPE_DHCP_MANAGER, NMDHCPManagerClass))