Add PEM_read_bio_ex

The extended function includes a 'flags' argument to allow callers
to specify different requested behaviors.  In particular, callers can
request that temporary storage buffers are allocated from the secure heap,
which could be relevant when loading private key material.

Refactor PEM_read_bio to use BIO_mems instead of BUFs directly,
use some helper routines to reduce the overall function length, and make
some of the checks more reasonable.

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