Correct many pre-1989 entries for Korea.

(Thanks to Sanghyuk Jung.)
* asia (ROK): Add entries for 1948-1959 and correct entries
for 1987-1988.
(Asia/Seoul): Correct entries through 1962.
(Asia/Pyongyang): Guess no change since World War II.
* NEWS: Document this.
Remove stray line in previous patch.
* leapseconds.awk: Update the IERS's full name.
Be less enthusiastic about Shanks and clarify UT vs UTC.

* africa, asia, australasia, backzone, europe, northamerica:
* southamerica, tz-link.htm: Be less enthusiastic about Shanks.
* tz-link.htm: Be clearer about UT vs UTC.
* NEWS: Document this.
Move out-of-scope UTC+3 African zones to 'backzone'.

* africa (Africa/Addis_Ababa, Africa/Asmara, Africa/Dar_es_Salaam)
(Africa/Djibouti, Africa/Kampala, Africa/Mogadishu, Indian/Antananarivo)
(Indian/Comoro, Indian/Mayotte):
Change these from zones to links to Africa/Nairobi.
* backzone: Move the old data here.
* checktab.awk (tztab): Remove special cases for these zones.
* NEWS: Document the above.
* europe: Add Countess Markievicz's take on the Summer Time Act, 1916.
Fix dates for yesterday's changes.
* tz-link.htm: Modernize URLs a bit.  Omit unnecessary trailing slashes.