BCP 47: duplicate U extension keywords are rejected as invalid by ICU4C. ICU4J is fine with it

Spun off from Ecma 402 bug 262 , ICU4J does allows duplicate keywords in U-extension of BCP 47 per RFC 6067 while ICU4C does not.

ICU4C's behavior should match ICU4J's. Note that RFC 6067 does allow duplicate keywords. The second or later one is ignored, while the first one is honored.

I can work on this.