Remove references to MoshClientInstance from SSHLogin.

This cleans up the tortured interface between these two classes. SSHLogin now
stands on its own, although it still uses the Pepper API (pp::Var, etc.).
Eliminate exit() calls.

Such calls look like crashes to the NaCl runtime, and cause throttling of
module startups, leading to confusion if a user connects and disconnects in
rapid succession (as happens during development).
Bump version to v0.2.3.24.