PEM: constify PEM_write_ routines

PEM: constify PEM_write_ routines

There's no reason why the object to be written, or the key string
given by the caller should be non-const.

This makes the IMPLEMENT_PEM_..._const and  DECLARE_PEM_..._const
macros superfluous, so we keep them around but mark them deprecated.

In all places where IMPLEMENT_PEM_..._const and  DECLARE_PEM_..._const
are used, they are replaced with the corresponding macros without
'_const'.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10452)