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/pkey/ecdh_curve25519.c b/programs/pkey/ecdh_curve25519.c
index 9f849dd..e79dce0 100644
--- a/programs/pkey/ecdh_curve25519.c
+++ b/programs/pkey/ecdh_curve25519.c
@@ -45,7 +45,7 @@
                     "MBEDTLS_ECP_DP_CURVE25519_ENABLED and/or "
                     "MBEDTLS_ENTROPY_C and/or MBEDTLS_CTR_DRBG_C "
                     "not defined\n" );
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -238,7 +238,7 @@
     mbedtls_ctr_drbg_free( &ctr_drbg );
     mbedtls_entropy_free( &entropy );
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_ECDH_C && MBEDTLS_ECP_DP_CURVE25519_ENABLED &&
           MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C */