util: add a steal_fd() helper function

We can't use steal for fds because of the -1 requirement.

Assisted-by: Claude:claude-opus-4-6
Part-of: <https://gitlab.freedesktop.org/libinput/libei/-/merge_requests/388>

src/util-mem.h

@@ -96,6 +96,21 @@ _steal(void *ptr)
 #define steal(ptr_) \
   (typeof(*ptr_))_steal(ptr_)
 
+/**
+ * Resets the pointer content to -1 and returns
+ * the original value.
+ */
+static inline int
+steal_fd(int *ptr)
+{
+	if (ptr) {
+		int original = *ptr;
+		*ptr = -1;
+		return original;
+	}
+	return -1;
+}
+
 /**
  * Never-failing calloc with a size limit check.
  */