HMAC_CTX_copy's documentation is off. It actually follows the old copy
functions which call FOO_init on dest first. Notably this means that they leak
memory if dest is currently in use.

Add HMAC_CTX_copy_ex as an analog of EVP_MD_CTX_copy and deprecate
HMAC_CTX_copy. (EVP_CIPHER_CTX_copy, in contrast, was correct from the start.)

Change-Id: I48566c858663d3f659bd356200cf862e196576c9
Reviewed-on: https://boringssl-review.googlesource.com/2694
Reviewed-by: Adam Langley <agl@google.com>