auditdistd: Use void pointers for opaque data

[des]

Summary

Use void, not char, for pointers to opaque data in our send and receive
functions.

Sponsored by: Klara, Inc.
Sponsored by: Qualys, Inc.

[emaste]

LGTM. The commit message should include *why* the change is being made though, not just *what* is being changed.