tree-wide: use -EBADF for fd initialization

-1 was used everywhere, but -EBADF or -EBADFD started being used in various
places. Let's make things consistent in the new style.

Note that there are two candidates:
EBADF 9 Bad file descriptor
EBADFD 77 File descriptor in bad state

Since we're initializating the fd, we're just assigning a value that means
"no fd yet", so it's just a bad file descriptor, and the first errno fits
better. If instead we had a valid file descriptor that became invalid because
of some operation or state change, the other errno would fit better.

In some places, initialization is dropped if unnecessary.

src/shared/switch-root.c

@@ -31,7 +31,7 @@ int switch_root(const char *new_root,
                 unsigned long mount_flags) {  /* MS_MOVE or MS_BIND */
 
         _cleanup_free_ char *resolved_old_root_after = NULL;
-        _cleanup_close_ int old_root_fd = -1;
+        _cleanup_close_ int old_root_fd = -EBADF;
         int r;
 
         assert(new_root);