Add AT_HANDLE_FID from Linux 6.5 to bits/fcntl-linux.h

Linux 6.5 adds a constant AT_HANDLE_FID; add it to glibc.  Because
this is a flag for the function name_to_handle_at declared in
bits/fcntl-linux.h, put the flag there rather than alongside other
AT_* flags in (OS-independent) fcntl.h.

Tested for x86_64.