Various tweaks to make things run on Mac OS.

The main change is the use of pthread to implement ffork.

src/lib9/sysfatal.c

@@ -14,6 +14,7 @@ sysfatal(char *fmt, ...)
 	vseprint(buf, buf+sizeof buf, fmt, arg);
 	va_end(arg);
 
+	__fixargv0();
 	fprint(2, "%s; %s\n", argv0 ? argv0 : "<prog>", buf);
 	exits("fatal");
 }