npppd: convert to EVP_MD_CTX on heap

npppd: convert to EVP_MD_CTX on heap

In the upcoming libcrypto bump, EVP_MD_CTX will become opaque, so
all EVP_MD_CTX variables will need to be moved from the stack to
the heap. This is a mechanical conversion which also switches
from EVP_Digest{Init,Final}() to their _ex() versions as suggested
by millert.

We cannot do error checking since this code is structured in
several layers of void functions. This will have to be fixed
by someone else.

ok millert