Support language tags in older ICU versions (53 and earlier).
By calling uloc_canonicalize() before parsing the attributes, the
existing locale attribute parsing logic works on language tags as
well.
Fix a small memory leak, too.
Discussion: http://postgr.es/m/
60da0cecfb512a78b8666b31631a636215d8ce73.camel@j-davis.com
Reviewed-by: Peter Eisentraut