stat-util: unify code that checks whether something is a regular file

Let's add a common implementation for regular file checks, that are
careful to return the right error code (EISDIR/EISLNK/EBADFD) when we
are encountering a wrong file node.

src/basic/stat-util.h

@@ -75,3 +75,6 @@ int path_is_temporary_fs(const char *path);
  * signed/unsigned comparison, because the magic can be 32 bit unsigned.
  */
 #define F_TYPE_EQUAL(a, b) (a == (typeof(a)) b)
+
+int stat_verify_regular(const struct stat *st);
+int fd_verify_regular(int fd);