Make OpenSSL an hcrypto backend proper

This adds a new backend for libhcrypto: the OpenSSL backend.

Now libhcrypto has these backends:

 - hcrypto itself (i.e., the algorithms coded in lib/hcrypto)
 - Common Crypto (OS X)
 - PKCS#11 (specifically for Solaris, but not Solaris-specific)
 - Windows CNG (Windows)
 - OpenSSL (generic)

The ./configure --with-openssl=... option no longer disables the use of
hcrypto.  Instead it enables the use of OpenSSL as a (and the default)
backend in libhcrypto.  The libhcrypto framework is now always used.

OpenSSL should no longer be used directly within Heimdal, except in the
OpenSSL hcrypto backend itself, and files where elliptic curve (EC)
crypto is needed.

Because libhcrypto's EC support is incomplete, we can only use OpenSSL
for EC.  Currently that means separating all EC-using code so that it
does not use hcrypto, thus the libhx509/hxtool and PKINIT EC code has
been moved out of the files it used to be in.

lib/hcrypto/bn.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2006 Kungliga Tekniska Högskolan
+ * Copyright (c) 2006-2016 Kungliga Tekniska Högskolan
  * (Royal Institute of Technology, Stockholm, Sweden).
  * All rights reserved.
  *
@@ -72,6 +72,8 @@
 #define BN_GENCB hc_BN_GENCB
 #define BN_CTX hc_BN_CTX
 #define BN_BLINDING hc_BN_BLINDING
+#define BN_MONT_CTX hc_BN_MONT_CTX
+
 
 /*
  *