Converting check_autoarg from a macro to a function lost the behavior. Instead,
just move the logic into p_rsa.c which was the only EVP_PKEY implementation
that even needed the flag.

Also document this behavior on each of the functions. Make note of the out =
NULL case only returning the maximum output size, and not necessarily the
actual size.

For testing, update example_sign to determine the signature size using the NULL
behavior rather than querying the RSA key.

Change-Id: Iec6c2862028a5cfdefe8faa0e8c471755070898a
Reviewed-on: https://boringssl-review.googlesource.com/1121
Reviewed-by: Adam Langley <agl@google.com>