Change the name for the new rs_context parameter.

Change the reserved name for the new rs_context parameter to be "context".

Since there can be only one rs_context parameter, report an  error if the
name is incorrect.

Also use "special parameter" consistently in the error messages to help
our developers distinguish between regular and special parameters.

Finally, some refactoring to make the code more understandable.

b/21647353

Change-Id: I6f23dc5a8ba3659714ad1f7ba41836ff8b92d9ec
18 files changed