bcc_usdt_foreach and bcc_usdt_foreach_uprobe should allow for user data

`bcc_usdt_foreach` and `bcc_usdt_foreach_uprobe` do not currently accommodate passing user data through to the callback, making it complicated to bind these interfaces in languages where closures are more complicated.

Start by locating the definitions and callback signatures for bcc_usdt_foreach and bcc_usdt_foreach_uprobe. Trace how each callback is invoked and determine how user data should pass through to it. Done means both interfaces accept user data and make it available to the callback without breaking existing callers.

Written by the indexing model from the issue text.