m_maphide Ignore unregistered users

Fixes #400 reported by @SaberUK
m_spanningtree Fix rare desync when a KILL crosses a message that has the killed user's prefix and modifies global state
Fix warning: "const type qualifier on return type has no effect".
Merge pull request #403 from SaberUK/insp20+wextra-fixes

Fix warning: "const type qualifier on return type has no effect".
Fix OpenSSL detection on distributions which modify the version number.
Merge pull request #407 from SaberUK/insp20+fix-openssl-detection

Fix OpenSSL detection on distributions which modify the version number.