Stage 1.0 CR fix: code passes "make check-spaces" with no warnings.
Stage 1.0: CR fix: stuff in src/common shouldn't include or use stuff in src/or

Details
- also removed the tor_assert call in sandbox.c for installing the filter (the purpose for this was that the function should only be called when debugging code);
- refactored check for Sandboxing command line option in main
Stage 1.0 CR fix: functions with no arguments should be defined accordingly
Stage 1.0 CR fix:  strip out the unused , #if 0 parts before merging.
Stage 1.0 CR fix: removed  single-point-of-return; temporary bug fix for accept4

- do{}while(0) constructs which allowed for a single point of return were removed and replaced either with return or goto
- a bug was identified within old code which was hiding a filter issue; accept4 cannot be applied in the filter due to a -33 error (EDOM = Math argument out of domain of func) which doesn't make sense at the moment; as a temporary fix I have replaced it with the socketcall syscall which allows all socket syscalls, including accept4;
Stage 1.0 CR fix: removed filters.h as it was not needed.

The general filter structure was moved in sandbox.c
Stage 1.0 Refactored out seccomp2.h, CR (fix) sigsys_debugging() now terminates application, CR (fix) now calling seccomp_rule_add() instead of _add_exact()

- seccomp2.h was no longer needed since we're using libseccomp instead of seccomp2