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/test/zeroize.c b/programs/test/zeroize.c
index 29cc0ac..6f57260 100644
--- a/programs/test/zeroize.c
+++ b/programs/test/zeroize.c
@@ -40,8 +40,10 @@
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
 #else
+#include <stdio.h>
 #include <stdlib.h>
 #define mbedtls_printf     printf
+#define mbedtls_exit       exit
 #define MBEDTLS_EXIT_SUCCESS EXIT_SUCCESS
 #define MBEDTLS_EXIT_FAILURE EXIT_FAILURE
 #endif
@@ -72,14 +74,14 @@
     {
         mbedtls_printf( "This program takes exactly 1 agument\n" );
         usage();
-        return( exit_code );
+        mbedtls_exit( exit_code );
     }
 
     fp = fopen( argv[1], "r" );
     if( fp == NULL )
     {
         mbedtls_printf( "Could not open file '%s'\n", argv[1] );
-        return( exit_code );
+        mbedtls_exit( exit_code );
     }
 
     while( ( c = fgetc( fp ) ) != EOF && p < end - 1 )
@@ -97,5 +99,5 @@
     fclose( fp );
     mbedtls_platform_zeroize( buf, sizeof( buf ) );
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }