Channels - explicit links between protocols and tables

The patch adds support for channels, structures connecting protocols and
tables and handling most interactions between them. The documentation is
missing yet.

lib/birdlib.h

@@ -34,6 +34,7 @@
 #define ABS(a)   ((a)>=0 ? (a) : -(a))
 #define DELTA(a,b) (((a)>=(b))?(a)-(b):(b)-(a))
 #define ARRAY_SIZE(a) (sizeof(a)/sizeof(*(a)))
+#define CALL(fn, args...) ({ if (fn) fn(args); })
 
 static inline int uint_cmp(uint i1, uint i2)
 { return (int)(i1 > i2) - (int)(i1 < i2); }