Update: Add unit tests for f_environment, make changes to f_environment, and add...

Update: Add unit tests for f_environment, make changes to f_environment, and add new function f_environment_secure_is().

Add the unit tests for f_environment.

The new function f_environment_secure_is() brings in libcap dependency requirements to f_environment.
This is added to add support for something similar to secure_getenv() rather than wrapping a non-standard method.
I am still not very experience with capabilities and do not guarantee an accurate replacement for secure_getenv().

Replace F_valid_not error returns with F_parameter error returns.