libthread: add threadmaybackground

Programs that want to background themselves now need
to define threadmaybackground returning 1.
This avoids a confusing (to people and debuggers)
extra parent process for all the threaded programs
that will never want to background themselves.

src/cmd/import.c

@@ -51,6 +51,12 @@ fatal(char *fmt, ...)
 	threadexitsall("fatal");
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char *argv[])
 {