Use OSSL_STORE for load_{,pub}key() and load_cert() in apps/lib/apps.c

Use OSSL_STORE for load_{,pub}key() and load_cert() in apps/lib/apps.c

This also adds the more flexible and general load_key_cert_crl()
as well as helper functions get_passwd(), cleanse(), and clear_free()
to be used also in apps/cmp.c etc.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/11755)