sudo: actually make the linting safety checks work

The current linting Execs, shamelessly stolen from the admin module, had
a logic error in which it tried doing "exit 0" as means to handle the
ensure => absent case, but ended up doing it unconditionally, therefore
rendering the whole check moot!

There's two ways to fix this: either move the "test -e" check to a
separate onlyif parameter, or guard the whole Exec with an "if $ensure
== present" check. The latter feels cleaner, so pick that one.

Change-Id: I9b29d3e093446cbd602087d27718f6813cfe372b