Refactor signal handling and replace signal handlers with thread-safe functions

[zzxuanyuan]

@bbockelm

1. I move signal handling from TSystem to a new class called TSigHandling and use gSigHandling as a global signal handling object in ROOT.
2. To comply with some function calls like "gSystem->ResetSignals()". I move old ResetSignals() to the new class so TUnixSystem::ResetSignals() just call gSigHandling->ResetSignals(). I could also go over all such function calls and replace them with gSigHandling->(functions) if necessary.
3. I replace old unsafe functions in signal handlers with thread-safe ones.
4. I only implement StackTrace functions for SIGBUS, SIGSEGV, SIGILL. Other signals are still using default StackTrace functions. kSigAlarm and kSigChild are ignored for my current implementation. Do we need to change other signal handlers?
5. @pcanal I have some problem with running roottest. I asked a question here:
   Fix issue ROOT-7588. #84
   Could you take a look at it and I will write test case this patch also.

[pcanal]

The ifndef CINT are no longer needed as it is (in v6) never defined