Add internal functions to fetch PEM data from an opened BIO

store_attach_pem_bio() creates a STORE_CTX with the 'file' scheme
loader backend in PEM reading mode on an already opened BIO.
store_detach_pem_bio() detaches the STORE_CTX from the BIO and
destroys it (without destroying the BIO).

These two functions can be used in place of STORE_open() and
STORE_close(), and are present as internal support for other OpenSSL
functions.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2745)