support --without-openssl at configure time

Disables and removes dependency on OpenSSL. Many features don't
work and the set of crypto options is greatly restricted. This
will only work on system with native arc4random or /dev/urandom.

Considered highly experimental for now.
diff --git a/sshd.c b/sshd.c
index d59e255..202e170 100644
--- a/sshd.c
+++ b/sshd.c
@@ -623,7 +623,9 @@
 
 	arc4random_stir();
 	arc4random_buf(rnd, sizeof(rnd));
+#ifdef WITH_OPENSSL
 	RAND_seed(rnd, sizeof(rnd));
+#endif
 	explicit_bzero(rnd, sizeof(rnd));
 
 	/* Demote the private keys to public keys. */
@@ -758,7 +760,9 @@
 
 	arc4random_stir();
 	arc4random_buf(rnd, sizeof(rnd));
+#ifdef WITH_OPENSSL
 	RAND_seed(rnd, sizeof(rnd));
+#endif
 	explicit_bzero(rnd, sizeof(rnd));
 
 	/* Drop privileges */
@@ -988,7 +992,7 @@
 #endif
 		buffer_put_int(&m, 0);
 
-#ifndef OPENSSL_PRNG_ONLY
+#if defined(WITH_OPENSSL) && !defined(OPENSSL_PRNG_ONLY)
 	rexec_send_rng_seed(&m);
 #endif
 
@@ -1041,7 +1045,7 @@
 #endif
 	}
 
-#ifndef OPENSSL_PRNG_ONLY
+#if defined(WITH_OPENSSL) && !defined(OPENSSL_PRNG_ONLY)
 	rexec_recv_rng_seed(&m);
 #endif
 
@@ -1372,7 +1376,9 @@
 			 */
 			arc4random_stir();
 			arc4random_buf(rnd, sizeof(rnd));
+#ifdef WITH_OPENSSL
 			RAND_seed(rnd, sizeof(rnd));
+#endif
 			explicit_bzero(rnd, sizeof(rnd));
 		}