Rename arguments of X509_STORE_CTX_init()

Rename arguments of X509_STORE_CTX_init()

It is higly confusing to call the list of untrusted certs chain, when
you're later going to call X509_STORE_CTX_get0_chain() to get a completely
unrelated chain by the verifier. Other X509_STORE_CTX APIs call this list
of certs 'untrusted', so go with that. At the same time, rename the x509
into leaf, which is more explicit.

suggested by/ok jsing