Try to unify BIO read/write parameter names

After the recent reworking, not everything matched up, and some
comments didn't catch up to the outl-->dlen and inl-->dlen renames
that happened during the development of the recent patches.

Try to make parameter names consistent across header, implementation,
and manual pages.

Also remove some trailing whitespace that was inadvertently introduced.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1798)