ktrace.1: Add ARGS and ENVS as default trace points

After commit 65a4daeaf324 ("ktrace: log execve(2) arguments and
environment"), "a" and "e" are part of the default set of trace points.

Reviewed by: markj
Approved by: emaste (mentor)
Fixes: 65a4daeaf324 ("ktrace: log execve(2) arguments and environment")
Differential Revision: https://reviews.freebsd.org/D47648