fileio: port write_string_file() to LabelOps, and thus add WRITE_STRING_FILE_LABEL flag

Given that we have the LabelOps abstraction these days, we can teach
write_string_file() to use it, which means we can get rid of
fileio-label.[ch] as a separate concept.

(The only reason that fileio-label.[ch] exists independently of
fileio.[ch] was that the former linekd to libselinux potentially, and
thus had to be in src/shared/ while the other always was in src/basic/.
But the LabelOps vtable provides us with a nice work-around)

src/basic/fileio.h

@@ -28,11 +28,7 @@ typedef enum {
         WRITE_STRING_FILE_MODE_0600                  = 1 << 10,
         WRITE_STRING_FILE_MODE_0444                  = 1 << 11,
         WRITE_STRING_FILE_SUPPRESS_REDUNDANT_VIRTUAL = 1 << 12,
-
-        /* And before you wonder, why write_string_file_atomic_label_ts() is a separate function instead of just one
-           more flag here: it's about linking: we don't want to pull -lselinux into all users of write_string_file()
-           and friends. */
-
+        WRITE_STRING_FILE_LABEL                      = 1 << 13,
 } WriteStringFileFlags;
 
 typedef enum {