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/hash/hello.c b/programs/hash/hello.c
index 2e8c224..3a3ed4c 100644
--- a/programs/hash/hello.c
+++ b/programs/hash/hello.c
@@ -31,6 +31,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #define mbedtls_printf       printf
+#define mbedtls_exit         exit
 #define MBEDTLS_EXIT_SUCCESS EXIT_SUCCESS
 #define MBEDTLS_EXIT_FAILURE EXIT_FAILURE
 #endif
@@ -43,7 +44,7 @@
 int main( void )
 {
     mbedtls_printf("MBEDTLS_MD5_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 int main( void )
@@ -55,7 +56,7 @@
     mbedtls_printf( "\n  MD5('%s') = ", str );
 
     if( ( ret = mbedtls_md5_ret( (unsigned char *) str, 13, digest ) ) != 0 )
-        return( MBEDTLS_EXIT_FAILURE );
+        mbedtls_exit( MBEDTLS_EXIT_FAILURE );
 
     for( i = 0; i < 16; i++ )
         mbedtls_printf( "%02x", digest[i] );
@@ -67,6 +68,6 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( MBEDTLS_EXIT_SUCCESS );
+    mbedtls_exit( MBEDTLS_EXIT_SUCCESS );
 }
 #endif /* MBEDTLS_MD5_C */