sql: don't mangle name of unique constraint

If UNIQUE constraint is specified in CREATE TABLE statement and it has
given name, one is mangled with pattern "unique_%s_%d", where %s is
original name of constraint and %d - current iid. For instance:

CREATE TABLE t (id INT PRIMARY KEY, a INT CONSTRAINT i1 UNIQUE);

This statement results in secondary index creation with name
"unique_I1_1". Justification for mangling is that constraint namespace
should be independent from index namespace. However, ALTER TABLE ADD
CONSTRAINT UNIQUE which is alias to CREATE INDEX doesn't mangle name.
What is more, users may wonder why name of index is different from
name of created constraint. Hence, it has been decided to remove this
mangling and create index exactly with specified constraint's name.