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/libthread/mkfile

@@ -4,6 +4,7 @@ SYSOFILES=`{sh ./sysofiles.sh}
 LIB=libthread.a
 OFILES=\
 	$SYSOFILES\
+	bg.$O\
 	channel.$O\
 	daemonize.$O\
 	exec.$O\