sysctl: Fix usage() and SYNOPSIS section of man page

I forgot to update the usage() and SYNOPSIS section of man page when
adding the two options -J and -V.

While here, fix the options sort order in SYNOPSIS section.

Fixes: 5ec83c660aca sysctl: Add flags to filter jail prison and vnet variables
MFC after: 1 day

(cherry picked from commit 0ee3e29e1d9ee3ed22441c0d580d2d1e57a1d9e3)
(cherry picked from commit 1d890d546b064f2c7753b949f5dbf00d79cde292)