basic: split out dev_t related calls into new devno-util.[ch]

No actual code changes, just splitting out of some dev_t handling
related calls from stat-util.[ch], they are quite a number already, and
deserve their own module now I think.

Also, try to settle on the name "devnum" as the name for the concept,
instead of "devno" or "dev" or "devid". "devnum" is the name exported in
udev APIs, hence probably best to stick to that. (this just renames a
few symbols to "devum", local variables are left untouched, to make the
patch not too invasive)

No actual code changes.

src/basic/devnum-util.h

@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <inttypes.h>
+#include <stdbool.h>
+#include <sys/types.h>
+
+int parse_devnum(const char *s, dev_t *ret);
+
+/* glibc and the Linux kernel have different ideas about the major/minor size. These calls will check whether the
+ * specified major is valid by the Linux kernel's standards, not by glibc's. Linux has 20bits of minor, and 12 bits of
+ * major space. See MINORBITS in linux/kdev_t.h in the kernel sources. (If you wonder why we define _y here, instead of
+ * comparing directly >= 0: it's to trick out -Wtype-limits, which would otherwise complain if the type is unsigned, as
+ * such a test would be pointless in such a case.) */
+
+#define DEVICE_MAJOR_VALID(x)                                           \
+        ({                                                              \
+                typeof(x) _x = (x), _y = 0;                             \
+                _x >= _y && _x < (UINT32_C(1) << 12);                   \
+                                                                        \
+        })
+
+#define DEVICE_MINOR_VALID(x)                                           \
+        ({                                                              \
+                typeof(x) _x = (x), _y = 0;                             \
+                _x >= _y && _x < (UINT32_C(1) << 20);                   \
+        })
+
+int device_path_make_major_minor(mode_t mode, dev_t devnum, char **ret);
+int device_path_make_canonical(mode_t mode, dev_t devnum, char **ret);
+int device_path_parse_major_minor(const char *path, mode_t *ret_mode, dev_t *ret_devnum);
+
+static inline bool devnum_set_and_equal(dev_t a, dev_t b) {
+        /* Returns true if a and b definitely refer to the same device. If either is zero, this means "don't
+         * know" and we'll return false */
+        return a == b && a != 0;
+}