util: add xclose(fd) to close-and-reset an fd

This prevents accidentally leaving the fd set after closing.

And it includes the -1 check so we don't need this everywhere ourselves
(not that we use it right now but valgrind likes to complain about
this).

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1067>

src/util-files.h

@@ -27,6 +27,7 @@
 
 #include <errno.h>
 #include <libgen.h>
+#include <unistd.h>
 #include <sys/stat.h>
 
 #include "util-strings.h"
@@ -52,3 +53,12 @@ mkdir_p(const char *dir)
 
 	return (rc == -1 && errno != EEXIST) ? -errno : 0;
 }
+
+static inline void
+xclose(int *fd)
+{
+	if (*fd != -1) {
+		close(*fd);
+		*fd = -1;
+	}
+}