udev,network: use standard paths for .network and .link files

This centralizes the configuration to one header file.
/usr/local/lib is now included in the search list, and documentation is
updated accordingly.

src/libsystemd-network/network-internal.h

@@ -8,7 +8,9 @@
 
 #include "condition.h"
 #include "conf-parser.h"
+#include "def.h"
 #include "set.h"
+#include "strv.h"
 
 #define LINK_BRIDGE_PORT_PRIORITY_INVALID 128
 #define LINK_BRIDGE_PORT_PRIORITY_MAX 63
@@ -54,3 +56,5 @@ int deserialize_dhcp_routes(struct sd_dhcp_route **ret, size_t *ret_size, size_t
 
 /* It is not necessary to add deserialize_dhcp_option(). Use unhexmem() instead. */
 int serialize_dhcp_option(FILE *f, const char *key, const void *data, size_t size);
+
+#define NETWORK_DIRS ((const char* const*) CONF_PATHS_STRV("systemd/network"))