Add support for "present" and "missing" (or equivalent meaning) modifiers

[atc0005]

The current design that I'm going to polish up for a v0.1.0 release is based on dual yes/no flags for an attribute. This means for a size check we have a warning threshold and a critical threshold. This makes sense.

However, for an existence check I also provided two flags, one to signify that a match is a WARNING state, another to signify that a CRITICAL state has been reached. That's not too bad, but now I'm ready to work in support for asserting the presence (or lack) of permissions for owner, group or other.

If I go with the existing design, this would mean 9 flags for signifying that a value should be present, and another 9 flags for signifying that a value should not be present. This is a bit much and likely not something that a sysadmin wants to have to spell out with 20+ flags per Nagios command definition.

Instead, we can have a base set of attribute flags and set of modifiers like so:

• present-warning
• present-critical
• missing-warning
• missing-critical

Current thinking:

Because the modifiers would only apply to existence checks we wouldn't need (or allow) more than one.

[atc0005]

The modifiers would apply to flags like these:

• username
• groupName
• exists
• owner-write
• owner-read
• owner-execute
• group-write
• group-read
• group-execute
• other-write
• other-read
• other-execute