Adds is-failed to join is-active and is-enabled.

I grabbed this one from the todo list. Most of the functionality was
already there for is-active. I just needed to make check_one_unit take
the states to check for as an argument instead of the hardcoded
"active" and "reloading".

is-failed will return 1 if none of the units given are failed. This is
different from is-active which will return 3 if none of the units
given are active. It returns 3 with this comment:
/* According to LSB: "program is not running" */
As that does not make sense when looking for failed units I simply
chose 1 instead.