check_file_age: support for --ignore-missing

Patch to provide for a -i | --ignore-missing option. This allows files that
don't exist to be monitored for without causing an error. The use case is:

* one process periodically pushes files to an "input" queue for a second
  process
* the second process, when running cleanly will "consume" (process and remove)
  these files

So the conditions to meet are:
1. If the file doesn't exist, that's fine (OK)
2. If the file exists, but is less than X seconds, that's fine (OK)
3. If the file exists, but greater than X seconds, no good (CRITICAL)

Since now check_file_age doesn't permit this behavior, as a non-existent file
is considered CRITICAL.

Test case was contributed by our lovely Holger.