multilocker: serialize conflicting parallel operations

For performance or workaround reasons we sometimes have to avoid doing
two conflicting operations at the same time, but we can still run any
number of non-conflicting operations in parallel.

MultiLocker (suggestions for a better class name welcome) blocks the
calling thread until there are no threads attempting to run a conflicting
operation.

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

lib/Makefile

@@ -13,6 +13,7 @@ CRUCIBLE_OBJS = \
 	extentwalker.o \
 	fd.o \
 	fs.o \
+	multilock.o \
 	ntoa.o \
 	path.o \
 	process.o \