Fix LOCK clause error message

[tangenta]

Index Issue: pingcap/tidb#11486

These are bad (incompatible) SQL cases:

alter table t lock = first
alter table t lock = start
alter table t lock = commit
alter table t lock = binlog

The body "LOCK" EqOpt identifier in LockClause should be used when the identifier is an unreserved keyword, and report

Error 1801: Unknown LOCK type 'first'

in the first example. However, TiDB parser reports:

Error 1064: You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 26 near "first"

Claim this subtask based on the comments below this issue.

[lauhg]

Let me fix it.