Unify the example programs' termination

This is done to account for platforms, for which we want custom behavior
upon the program termination, hence we call `mbedtls_exit()` instead of
returning from `main()`.
diff --git a/programs/ssl/dtls_server.c b/programs/ssl/dtls_server.c
index bb32724..9121c34 100644
--- a/programs/ssl/dtls_server.c
+++ b/programs/ssl/dtls_server.c
@@ -29,6 +29,7 @@
 #include "mbedtls/platform.h"
 #else
 #include <stdio.h>
+#include <stdlib.h>
 #define mbedtls_printf     printf
 #define mbedtls_fprintf    fprintf
 #define mbedtls_time_t     time_t
@@ -61,7 +62,7 @@
             "MBEDTLS_X509_CRT_PARSE_C and/or MBEDTLS_RSA_C and/or "
             "MBEDTLS_CERTS_C and/or MBEDTLS_PEM_PARSE_C and/or "
             "MBEDTLS_TIMING_C not defined.\n" );
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -430,7 +431,7 @@
     if( ret < 0 )
         ret = 1;
 
-    return( ret );
+    mbedtls_exit( ret );
 }
 #endif /* MBEDTLS_SSL_SRV_C && MBEDTLS_SSL_PROTO_DTLS &&
           MBEDTLS_SSL_COOKIE_C && MBEDTLS_NET_C && MBEDTLS_ENTROPY_C &&