ndp: fix timestamp display output

The current xo_format string is incorrect. This restores the display
format prior to libxo-ification work while also explicitly marking
tv_sec and tv_usec as encoded output only.

MFC after: 1 week
Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42269