pki_crypto: Use the new OpenSSL API to read PEM files
This allows the use of the new PKCS#8 PEM files and does not
limit libssh to using only the "traditional" PEM files, that
are not default in OpenSSL since 1.0.0 and which do not work
at all in FIPS mode.
Signed-off-by: Jakub Jelen <firstname.lastname@example.org>
Reviewed-by: Andreas Schneider <email@example.com>