Check for links to links.

This problem has come up before, and it's time to add a check to
catch this common error.
* Makefile (AWK_SCRIPTS): Add checklinks.awk.
(check, .PHONY): Add check_links.
(check_links): New rule.
* checklinks.awk: New file.
Fix link-to-link for Asmera.

Problem reported by Stephen Colebourne in:
http://mm.icann.org/pipermail/tz/2014-November/021888.html
* backward (Africa/Asmera): Link to Africa/Nairobi,
not to Africa/Asmara.
* backzone (Africa/Asmera): New link, to override 'backward'.
* NEWS: Document fixes.
tzselect: Fix for gawk treating '\.' as plain '.'

When using gawk and Posix TZ in tzselect, this warning is shown:
	awk: cmd. line:1: warning: escape sequence `\.' treated as plain `.'
gawk treats '\.' as '.'
mawk treats '\.' as '\.'
both treat '\\.' as '\\.'
The 2nd and 3rd are corrrect for 'Mm.w.d'.

Test:
gawk 'BEGIN{ date = "\."; print date }'
gawk 'BEGIN{ date = "\\."; print date }'
versus:
mawk 'BEGIN{ date = "\."; print date }'
mawk 'BEGIN{ date = "\\."; print date }'
* tzselect.ksh (date): Fix bug that broke POSIX TZ checking with gawk.
* NEWS: Document this.
Adjust pre-1940 Iceland data to match the Iceland Almanak.

* europe (Iceland, Atlantic/Reykjavik):
Correct entries for 1939, 1921, 1919, and 1837-1907.
Coalesce some other entries, for simplicity.
* NEWS: Document this.
* NEWS: Fix misspelling.  (Thanks to Chris Rorvick.)
* europe (Iceland): Typo fix in commentary.