make debug() output more useful

This change does several things. First, and most important, it allows
dumping the "n" most recent stack frames on each debug() call. Second,
it demangles the C++ symbols. Third, it prepends each debug() message
with the debug level.

Unrelated to the above I've replaced all `assert(!is_forked_child());`
statements with `ASSERT_IS_NOT_FORKED_CHILD()` for consistency.

src/exec.cpp

@@ -948,7 +948,7 @@ void exec_job(parser_t &parser, job_t *j) {
                             bool builtin_io_done = do_builtin_io(outbuff.data(), outbuff.size(),
                                                                  errbuff.data(), errbuff.size());
                             if (!builtin_io_done && errno != EPIPE) {
-                                show_stackframe();
+                                show_stackframe(L'E');
                             }
                             fork_was_skipped = true;
                         }