MySQL 5.1 server

Server used to clip overly long user-names. This was presumably lost
when code was made UTF8-clean.

Now we emulate the behaviour for backward compatibility, but UTF8-ly
correct.


mysql-test/r/connect.result:
  Show that user-names that are too long get clipped now.
mysql-test/t/connect.test:
  Show that user-names that are too long get clipped now.
sql/sql_connect.cc:
  Clip user-name to 16 characters (not bytes).
strings/CHARSET_INFO.txt:
  Clarify in docs.