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/ecdsa.c b/programs/pkey/ecdsa.c
index b851c31..b898b62 100644
--- a/programs/pkey/ecdsa.c
+++ b/programs/pkey/ecdsa.c
@@ -66,7 +66,7 @@
 {
     mbedtls_printf("MBEDTLS_ECDSA_C and/or MBEDTLS_SHA256_C and/or "
            "MBEDTLS_ENTROPY_C and/or MBEDTLS_CTR_DRBG_C not defined\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 #if defined(VERBOSE)
@@ -248,7 +248,7 @@
     mbedtls_ctr_drbg_free( &ctr_drbg );
     mbedtls_entropy_free( &entropy );
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_ECDSA_C && MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C &&
           ECPARAMS */