Promote all SSL callback ctrl hooks to proper functions.

Document them while I'm here. This adds a new 'preprocessor
compatibility section' to avoid breaking #ifdefs. The CTRL values
themselves are defined to 'doesnt_exist' to catch anything calling
SSL_ctrl directly until that function can be unexported completely.

BUG=404754

Change-Id: Ia157490ea8efe0215d4079556a0c7643273e7601
Reviewed-on: https://boringssl-review.googlesource.com/4553
Reviewed-by: Adam Langley <agl@google.com>
8 files changed