Add PEM_bytes_read_bio_secmem()

Split the PEM_bytes_read_bio() implementation out into a
pem_bytes_read_bio_flags() helper, to allow it to pass PEM_FLAG_SECURE
as needed.  Adjust the cleanup to properly use OPENSSL_secure_free()
when needed, and reimplement PEM_bytes_read() as a wrapper around
the _flags helper.

Add documentation for PEM_bytes_read_bio() and the new secmem variant.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1700)