seeker: add a runtime debug stream

This allows detailed but selective debugging when using the library,
particularly when something goes wrong.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/Makefile

@@ -17,6 +17,7 @@ CRUCIBLE_OBJS = \
 	openat2.o \
 	path.o \
 	process.o \
+	seeker.o \
 	string.o \
 	table.o \
 	task.o \