Former Rep. Mark Foley, the disgraced Florida lawmaker who quit last week after revelations about explicit messages he sent to a teenage former House page surfaced, has begun seeking treatment at a rehabilitation facility for alcoholism.   On October 3, Foley's lawyer said that a clergyman had molested his client between ages 13 and 15, and confirmed for the fist time that Foley is gay.  Rev. Anthony Mercieca, the 69 year old retired priest from Malta acknowledged on October 19 that he had intimate contact with Foley that involved nudity and on at least one occasion, light touching, but denied that he and Foley had sexual intercourse.  The Roman Catholic diocese of Gozo, a small Mediterranean island off Malta, has opened an investigation of Mercieca, who said he fondled and shared saunas while naked with Foley when he was a boy.  Meanwhile, a former House page says he had sex with then-Rep. Foley after receiving explicit e-mails in 2000, which the congressman described assessing the sexual orientation and physical attributes of underage pages but waited until later to make direct advances.