Of all the hazards that young people face at college, here is one that probably crossed few parents’ minds: the easy access to tanning beds, often found in off-campus housing and sometimes paid for with university-sponsored cash cards. Now, cancer-prevention advocates are targeting colleges in a campaign to unplug indoor tanning, citing studies that link the practice to an increase in melanoma among young people. And colleges, including in New England, are beginning to change policies.