Make field visibility settings option ids unique

The 'id' attribute of xprofile visibility settings options was being
concatenated in such a way that there were sometimes conflicts between
different fields. Besides being bad practice, this also caused occasional
problems where changing one field's setting would inadvertently change
another's, due to browser behavior regarding duplicate ids.

Fixes #5248

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7617 cdf35c40-ae34-48e0-9cc9-0c9da1808c22