Before WL#7083, ENFORCE_GTID_CONSISTENCY was a boolean.  Now it is a
three-valued variable.  Normally a three-valued variable would be of
type Sys_var_enum.  However, Sys_var_enum is incompatible with boolean
in the following ways:

- When the variable is SELECT'ed, boolean shows a numeric value (0 or
  1) whereas enum shows the textual alias.

- Booleans have two textual aliases for each value (0=off=false,
  1=on=true), whereas enum only allows one.

- The command-line option for a boolean can be used without an
  argument: '--command-line-option' without '=VALUE', which has the
  meaning 'TRUE'.  If an enum allows the argument to be optional, it
  takes the default value, which in our case is different from TRUE.

In order to avoid breaking compatibility with this behavior, we
introduce a new Sys_var class for enumeration-like variables that can
have multiple aliases, are displayed as integers, and allow
'--command-line-option' without '=VALUE' to have a different value
than the default value.

@sys_vars.h
- Add new class Sys_var_int_with_aliases.
- Make Sys_var_enforce_gtid_consistency derive from the new class.

@sys_vars.cc
- Make Sys_var_enforce_gtid_consistency derive from the new class.