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/basic/util.h

@@ -176,8 +176,6 @@ static inline unsigned log2u_round_up(unsigned x) {
         return log2u(x - 1) + 1;
 }
 
-bool id128_is_valid(const char *s) _pure_;
-
 int container_get_leader(const char *machine, pid_t *pid);
 
 int namespace_open(pid_t pid, int *pidns_fd, int *mntns_fd, int *netns_fd, int *userns_fd, int *root_fd);