The problem was that appending values to the end of an existing
ENUM or SET column was being treated as table data modification,
preventing a immediately (fast) table alteration that occurs when
only table metadata is being modified.

The cause was twofold: adding a enumeration or set members to the 
end of the list of valid member values was not being considered
a "compatible" table alteration, and for SET columns, the check
was being done upon the max display length and not the underlying
(pack) length of the field.

The solution is to augment the function that checks wether two ENUM
or SET fields are compatible -- by comparing the pack lengths and
performing a limited comparison of the member values.

mysql-test/r/alter_table.result:
  Add test case result for Bug#45567
mysql-test/t/alter_table.test:
  Add test case for Bug#45567
sql/field.cc:
  Check whether two fields can be considered 'equal' for table
  alteration purposes. Fields are equal if they retain the same
  pack length and if new members are added to the end of the list.
sql/field.h:
  Add comment and remove method.