apps/speed.c: add -aead flag.

Goal is to exercise AEAD ciphers in TLS-like sequence, i.e. 13-byte
AAD followed by payload. Update doc/man1/speed.pod accordingly.

[While we are at it, address even some styling and readability issues.]

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6311)