Fork to execute systemctl calls
Calling os.execute() to apply presets was a mistake for two reasons:
1) os.execute() calls out to a shell, which defeated the purpose of
rewriting the scriptlet in Lua to avoid a /bin/sh dependency.
2) os.execute() only took a single argument, so it wasn't actually
applying the presets at all.
Rewriting this to explicitly fork()/exec() results in properly-
applied presets without the /bin/sh dependency.
Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>