Add support for generating backtraces on Windows

This commit adds support for generating backtraces on Windows and refactors the isc_backtrace API to closely match the Linux/BSD API:

• isc_backtrace_gettrace() was renamed to isc_backtrace()
• isc_backtrace_symbols() was added
• isc_backtrace_symbols_fd() was added and used as appropriate

See #2638 (closed)