etcupdate: fix arguments order of diff command

Due to misplacement of the second -L argument of diff command, it is
treated by getopt_long as an error.

Also add -l option for a diff command that alters the way it shows
differences. Instead of printing full diff, it reports changed file
the same way as added/removed files are reported.

Reviewed by: imp
Obtained from: Yandex LLC
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56708