util: split out extract_first_word() and related calls into extract-word.[ch]

This is quite a lot of code these days, hence move it to its own source
file.

src/basic/cgroup-util.c

@@ -19,27 +19,28 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <errno.h>
-#include <unistd.h>
-#include <signal.h>
-#include <string.h>
-#include <stdlib.h>
 #include <dirent.h>
+#include <errno.h>
+#include <ftw.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
-#include <ftw.h>
+#include <unistd.h>
 
-#include "set.h"
-#include "macro.h"
-#include "util.h"
-#include "formats-util.h"
-#include "process-util.h"
-#include "path-util.h"
-#include "unit-name.h"
+#include "extract-word.h"
 #include "fileio.h"
-#include "special.h"
-#include "mkdir.h"
+#include "formats-util.h"
 #include "login-util.h"
+#include "macro.h"
+#include "mkdir.h"
+#include "path-util.h"
+#include "process-util.h"
+#include "set.h"
+#include "special.h"
+#include "unit-name.h"
+#include "util.h"
 #include "cgroup-util.h"
 
 int cg_enumerate_processes(const char *controller, const char *path, FILE **_f) {