apps/ca,req,x509: Switch to EVP_DigestSignInit_ex

apps/ca,req,x509: Switch to EVP_DigestSignInit_ex

Switch lib/apps.c do_sign_init() to use EVP_DigestSignInit_ex, so it
works with external providers.
Since EVP_DigestSignInit_ex requires a digest name instead of
an EVP_MD pointer, the apps using do_sign_init() had to be modified
to pass char* instead of EVP_MD*.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/15014)