Change test-varargs to check libunwind backtracing

test-varargs is checking how `backtrace()' provided by the system
behaves when varargs are used. Let's make the test more useful by
changing it to test the `backtrace()' provided by libunwind.

Change the testcase to return 0/1 for success/failure, and add it to the
set of checks, so that it gets run on `make check'. Also call
`unw_backtrace()' explicitly so that we do not need to bother with
`execinfo.h' and `backtrace()' prototype.
3 files changed