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.
The idea is to use the simple program that is expected to be modified
rarely to set a breakpoint in a specific line and check that the
function mbedtls_zeroize() does actually set the buffer to 0 and is not
optimised out by the compiler.
