fd-util: add new helper move_fd() and make use of it

We are using the same pattern at various places: call dup2() on an fd,
and close the old fd, usually in combination with some O_CLOEXEC
fiddling. Let's add a little helper for this, and port a few obvious
cases over.

src/basic/fd-util.h

@@ -75,6 +75,8 @@ bool fdname_is_valid(const char *s);
 
 int fd_get_path(int fd, char **ret);
 
+int move_fd(int from, int to, int cloexec);
+
 /* Hint: ENETUNREACH happens if we try to connect to "non-existing" special IP addresses, such as ::5 */
 #define ERRNO_IS_DISCONNECT(r) \
         IN_SET(r, ENOTCONN, ECONNRESET, ECONNREFUSED, ECONNABORTED, EPIPE, ENETUNREACH)