Add missing getSecureSince() function in ZIDRecordFile.h

Some changes in inline comments.
Add #ifdef to address compiler issues if not using GNUC
Add EnumBase destructor code to delete allocated data

Macrus Froeschl provided this code.
Fix a possible memset (clear memory) vulnerability

After a code review I got the following feedback, pointing out a possible
vulnerability when clearing memory:
The usage of the free() statement directly after a memset() statement. Such
sequence might be recognized and removed by a compiler in case of
optimisation.

I reviewed the ZRTP code and found some places that could be affected by this
compiler optimization. I fixed these memset calls using a specific call
seqeuence to avoid this sort of optimization.
Small fixes to remove some compiler warning in clang/LLVM
Add NULL pointer checks, fixes to remove compiler warnings in clang, MSC

Thanks to Marcus Froeschl for catching the NULL pointer cheks and testing the
compiler warning issues.
Some more fixes to remove compiler warnings in clang, MSC

Remove unused fields, some modifications to use "true", "false" instead of a
integer containing a zero or not zero value.
TIVI - Add security discriminator mode
Prepare V4.3.1