It was possible to crash a mysqld build with EXTRA_DEBUG using
CREATE TABLE ... COMMENT with a specially-crafted UTF-8 string.

This CS removes the check that caused it since it no longer
applies in current servers anyway, and adds comments instead
to avoid future confusion. 

mysql-test/r/strict.result:
  Try to crash mysqld with a "suitable" multi-byte (3-byte UTF-8) string
  for a table comment.
mysql-test/t/strict.test:
  Try to crash mysqld with a "suitable" multi-byte (3-byte UTF-8) string
  for a table comment.
sql/unireg.cc:
  Talk at length about limits in .frm form-info, characters vs bytes,
  inlined vs extra-segement TABLE-COMMENTS, and the differences in
  6.0+ vs <6.0 when it comes to UTF-8.
  
  Also, remove a check that no longer applies and that could lead to
  problems in pathological cases.