Eliminate key press requirement "show vmopag" command output.

Summary:
One was required to press a key to continue after every 18 lines of
output. This requirement had been in the "show vmopag" command since it
was introduced, which was many years before paging was added to DDB.
With paging, this explict key check is no longer necessary.

Obtained from: Juniper Networks, Inc.
MFC after: 1 week

Test Plan:
Run "show vmopag" from db> prompt and see that it does not need additional
keypresses other than the ones needed for the pager.

Subscribers: imp, Contributor Reviews (src)

Differential Revision: https://reviews.freebsd.org/D33550