[plugins] Update plugins to only use new consolidated methods

Updates numerous plugins to no longer use `get_command_output()`,
`call_ext_prog()`, or `check_ext_prog()` - instead only using
`collect_cmd_output()` where the content needs to be saved to the
archive, and `exec_cmd()` where it does not.

For the vast majority of plugins, these calls were able to be changed
in-place with little to no further alteration of plugin code outside of
no longer making indenpendent open() calls to files created, as the
content of those files is now directly available. Exceptions are noted
below.

For the dlm, gfs2, and kvm plugins the only use of get_command_output()
was to mount /sys/kernel/debugfs which in modern distributions is no
longer needed, so those plugins have had that functionality entirely
removed instead of being ported to the new `exec_cmd()`.

For the networking plugin, many of the checks are handled by
SoSPredicates, and so those command collections now use Predicates
rather than being gated by individual `exec_cmd()` calls.

Resolves: #1807

Signed-off-by: Jake Hunsaker <jhunsake@redhat.com>
Signed-off-by: Bryn M. Reeves <bmr@redhat.com>

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