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/x509/cert_req.c b/programs/x509/cert_req.c
index f3d9157..81e0d8c 100644
--- a/programs/x509/cert_req.c
+++ b/programs/x509/cert_req.c
@@ -46,7 +46,7 @@
             "MBEDTLS_PK_PARSE_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
 
@@ -444,7 +444,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_X509_CSR_WRITE_C && MBEDTLS_PK_PARSE_C && MBEDTLS_FS_IO &&
           MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C && MBEDTLS_PEM_WRITE_C */