fd-util: use F_DUPFD_QUERY for same_fd()

Catch up with the nice little toys the kernel fs developers have added
for us. Preferably, let's make use of the new F_DUPFD_QUERY fcntl() call
that checks whether two fds are just duplicates of each other
(duplicates as in dup(), not as in open() of the same inode, i.e.
whether they share a single file offset and so on).

This API is much nicer, since it is a core kernel feature, unlike the
kcmp() call we so far used, which is part of the (optional)
checkpoint/restore stuff.

F_DUPFD_QUERY is available since kernel 6.10.

src/basic/missing_fcntl.h

@@ -7,6 +7,10 @@
 #define F_LINUX_SPECIFIC_BASE 1024
 #endif
 
+#ifndef F_DUPFD_QUERY
+#define F_DUPFD_QUERY (F_LINUX_SPECIFIC_BASE + 3)
+#endif
+
 #ifndef F_SETPIPE_SZ
 #define F_SETPIPE_SZ (F_LINUX_SPECIFIC_BASE + 7)
 #endif