sd-id128: split UUID file read/write code into new id128-util.[ch]

We currently have code to read and write files containing UUIDs at various
places. Unify this in id128-util.[ch], and move some other stuff there too.

The new files are located in src/libsystemd/sd-id128/ (instead of src/shared/),
because they are actually the backend of sd_id128_get_machine() and
sd_id128_get_boot().

In follow-up patches we can use this reduce the code in nspawn and
machine-id-setup by adopted the common implementation.

src/nspawn/nspawn.c

@@ -61,6 +61,7 @@
 #include "fs-util.h"
 #include "gpt.h"
 #include "hostname-util.h"
+#include "id128-util.h"
 #include "log.h"
 #include "loopback-setup.h"
 #include "machine-id-setup.h"
@@ -76,10 +77,10 @@
 #include "nspawn-network.h"
 #include "nspawn-patch-uid.h"
 #include "nspawn-register.h"
+#include "nspawn-seccomp.h"
 #include "nspawn-settings.h"
 #include "nspawn-setuid.h"
 #include "nspawn-stub-pid1.h"
-#include "nspawn-seccomp.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "process-util.h"