crypt: Introduce crypt_shutdown().

crypt: Introduce crypt_shutdown().

This plugs a few harmless memory leaks in the openssl crypto
backend. The leaks occur on exit and are only reported by valgrind
if it is run with --leak-check=full --show-leak-kinds=all.

The gcrypt backend has similar problems, but there is no way to
provide a similar patch for libgrypt. The newly added comment in
gcrypt.c explains why.