This commit adds support for generating backtraces on Windows and refactors the isc_backtrace API to match the Linux/BSD API (without the isc_ prefix) * isc_backtrace_gettrace() was renamed to isc_backtrace(), the third argument was removed and the return type was changed to int * isc_backtrace_symbols() was added * isc_backtrace_symbols_fd() was added and used as appropriate