util: Don't use errno after a successful call in __daemon()

I thought we could just set errno to 0, do a bunch of stuff, and check
that errno didn't change to infer we succeeded. But clang-tidy,
starting with LLVM 19, reports:

/home/sbrivio/passt/util.c:465:6: error: An undefined value may be read from 'errno' [clang-analyzer-unix.Errno,-warnings-as-errors]
  465 |         if (errno)
      |             ^
/usr/include/errno.h:38:16: note: expanded from macro 'errno'
   38 | # define errno (*__errno_location ())
      |                ^~~~~~~~~~~~~~~~~~~~~~
/home/sbrivio/passt/util.c:446:6: note: Assuming the condition is false
  446 |         if (pid == -1) {
      |             ^~~~~~~~~
/home/sbrivio/passt/util.c:446:2: note: Taking false branch
  446 |         if (pid == -1) {
      |         ^
/home/sbrivio/passt/util.c:451:6: note: Assuming 'pid' is 0
  451 |         if (pid) {
      |             ^~~
/home/sbrivio/passt/util.c:451:2: note: Taking false branch
  451 |         if (pid) {
      |         ^
/home/sbrivio/passt/util.c:463:2: note: Assuming that 'close' is successful; 'errno' becomes undefined after the call
  463 |         close(devnull_fd);
      |         ^~~~~~~~~~~~~~~~~
/home/sbrivio/passt/util.c:465:6: note: An undefined value may be read from 'errno'
  465 |         if (errno)
      |             ^
/usr/include/errno.h:38:16: note: expanded from macro 'errno'
   38 | # define errno (*__errno_location ())
      |                ^~~~~~~~~~~~~~~~~~~~~~

And the LLVM documentation for the unix.Errno checker, 1.1.8.3
unix.Errno (C), mentions, at:

  https://clang.llvm.org/docs/analyzer/checkers.html#unix-errno

that:

  The C and POSIX standards often do not define if a standard library
  function may change value of errno if the call does not fail.
  Therefore, errno should only be used if it is known from the return
  value of a function that the call has failed.

which is, somewhat surprisingly, the case for close().

Instead of using errno, check the actual return values of the calls
we issue here.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

util.c

@@ -444,16 +444,11 @@ int __daemon(int pidfile_fd, int devnull_fd)
 		exit(EXIT_SUCCESS);
 	}
 
-	errno = 0;
-
-	setsid();
-
-	dup2(devnull_fd, STDIN_FILENO);
-	dup2(devnull_fd, STDOUT_FILENO);
-	dup2(devnull_fd, STDERR_FILENO);
-	close(devnull_fd);
-
-	if (errno)
+	if (setsid()				< 0 ||
+	    dup2(devnull_fd, STDIN_FILENO)	< 0 ||
+	    dup2(devnull_fd, STDOUT_FILENO)	< 0 ||
+	    dup2(devnull_fd, STDERR_FILENO)	< 0 ||
+	    close(devnull_fd))
 		exit(EXIT_FAILURE);
 
 	return 0;