rcs_commit and rcs_commit_staged api changes

Using named parameters for these is overdue. Passing the session in a
parameter instead of passing username and IP separately will later allow
storing other session info, like username or part of the email.

Note that these functions are not part of the exported API,
and the prototype change will catch (most) skew, so I am not changing
API versions. Any third-party plugins that call them will need updated
though.