Bugfix: recently added capability functions are from newer version.

Bugfix: recently added capability functions are from newer version.

The libcap project hasn't been updated in a long time.
Apparently, somebody relatively recently picked up the project and started maintaining it.
This introduced newer functions.

For some reason, my system has a hybrid of this.
The headers show the newer functions but the libraries lack them.

Add a new define "_libcap_legacy_only_".
Enable this by default given how long libcap has exist unchanged.

I also noticed and inconsistency with the function names for users and groups (which are newer functions).
Rename them to not include the "_id".
Remove a duplicate function that didn't even have a reference in the header (oops!).