shared: split out UID allocation range stuff from user-record.h

user-record.[ch] are about the UserRecord JSON stuff, and the UID
allocation range stuff (i.e. login.defs handling) is a very different
thing, and complex enough on its own, let's give it its own c/h files.

No code changes, just some splitting out of code.

src/sysusers/sysusers.c

@@ -31,8 +31,8 @@
 #include "strv.h"
 #include "sync-util.h"
 #include "tmpfile-util-label.h"
+#include "uid-alloc-range.h"
 #include "uid-range.h"
-#include "user-record.h"
 #include "user-util.h"
 #include "utf8.h"
 #include "util.h"