Use memset_s (or equivalent) when handling sensitive info

[rajivshah3]

Changes memset to a custom function when clearing sensitive info out of the memory. According to MSC06-C. Beware of compiler optimizations, memset could be removed by a compiler during optimization if it does not deem it necessary. memset_s and SecureZeroMemory are guaranteed to not be removed by the compiler (see https://en.cppreference.com/w/c/string/byte/memset and https://msdn.microsoft.com/en-us/library/windows/desktop/aa366877).

I changed common/helpers/sign.c and mobile/android/Interface.cpp, but I'm not sure if the memsets that are used in other places need to be changed as well.

Test Plan:

• Existing tests should pass

[thibault-martinez]

Some comments we had in private:

• memset_s is C11
• The availability of certain C11 features varies from one compiler to another
• Not available on Windows plateforms
• We can create a common entry point that would redirect to the most efficient available solution: memset_s if available, SecureZeroMemory if windows, custom function otherwise

[rajivshah3]

@thibault-martinez I changed this so that it will use whatever is available based on the preprocessor directives

[rajivshah3]

Closing in favor of #839