pg_trgm's set_limit() now uses SetConfigOption()

pg_trgm's set_limit() now uses SetConfigOption()

Deprecated set_limit() is modified to use SetConfigOption() to set
similarity_threshold which is actually an instance of
pg_trgm.similarity_threshold GUC variable. Previous coding directly sets
similarity_threshold what could cause an inconsistency between states of
actual variable and GUC representation.

Per gripe from Tom Lane