Fix postinstall script not removing watchdog-mux.socket

watchdog-mux.socket was removed in f8a3fc80af but the
postinstall script used -e instead of -L to test for the
symlink, which fails since the destination is already
removed at that point.

Use -L and remove the dead symlink if it exists.

Reported-by: Alexandre Derumier <aderumier@odiso.com>