Allow setting an absent => X config param for file checks.  If set will return all the metrics the module normally would, but with their integer values set to whatever you defined for absent.  present will still = 0 and text values will be undef

Replaces the following modules:

FILEAGE
FILESIZE

git-svn-id: https://labs.omniti.com/resmon/branches/resmon2@349 8c0face9-b7db-6ec6-c4b3-d5f7145c7d55