Generate the 'leapseconds' file automatically from leap-seconds.list.

* leap-seconds.list: New file, copied from the NIST.
* leapseconds: Remove from git; it's now generated.
* leapseconds.awk: New file.
* Makefile (DATA): Add leap-seconds.list.  Remove leapseconds.
(MISC): Add leapseconds.awk.
(leapseconds): New rule.
(right_posix, posix_right): Depend on leapseconds.
(clean_misc): Remove leapseconds.