basic,shared: move a bunch of files to src/shared/

The goal is to move everything that requires selinux or smack
away from src/basic/. This means that src/basic/label.[ch] must move,
which implies btrfs-util.[ch], copy.[ch], and a bunch of other files
which form a cluster of internal use.

This is just moving text around, so there should be no functional difference.

test-blockdev-util is new, because path_is_encrypted() is moved to
blockdev-util.c, and so far we didn't have any tests for code there.

src/libsystemd/sd-bus/bus-socket.c

@@ -22,7 +22,6 @@
 #include "path-util.h"
 #include "process-util.h"
 #include "rlimit-util.h"
-#include "selinux-util.h"
 #include "signal-util.h"
 #include "stdio-util.h"
 #include "string-util.h"