Pass parameters to tls1_process_sigalgs as a CBS.

Slightly cleaner; it means we can use CBS_stow.

Change-Id: I074aa2d73a79648013dea025ee531beeea2af4a2
Reviewed-on: https://boringssl-review.googlesource.com/1287
Reviewed-by: Adam Langley <agl@google.com>