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()`.

For the sake of consistency, introduces the modifications have been made
to the test and utility examples as well. These, while less likely to be
used in the low level environments, won't suffer from such a change.
diff --git a/programs/ssl/dtls_client.c b/programs/ssl/dtls_client.c
index c29ab34..b72981c 100644
--- a/programs/ssl/dtls_client.c
+++ b/programs/ssl/dtls_client.c
@@ -29,8 +29,10 @@
 #include "mbedtls/platform.h"
 #else
 #include <stdio.h>
+#include <stdlib.h>
 #define mbedtls_printf     printf
 #define mbedtls_fprintf    fprintf
+#define mbedtls_exit       exit
 #endif
 
 #if !defined(MBEDTLS_SSL_CLI_C) || !defined(MBEDTLS_SSL_PROTO_DTLS) ||    \
@@ -45,7 +47,7 @@
             "MBEDTLS_ENTROPY_C and/or MBEDTLS_CTR_DRBG_C and/or "
             "MBEDTLS_X509_CRT_PARSE_C and/or MBEDTLS_RSA_C and/or "
             "MBEDTLS_CERTS_C and/or MBEDTLS_PEM_PARSE_C not defined.\n" );
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -351,7 +353,7 @@
     if( ret < 0 )
         ret = 1;
 
-    return( ret );
+    mbedtls_exit( ret );
 }
 #endif /* MBEDTLS_SSL_CLI_C && MBEDTLS_SSL_PROTO_DTLS && MBEDTLS_NET_C &&
           MBEDTLD_TIMING_C && MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C &&