Modifying the System.getEnv function to return IO (Maybe String)

For edwinb/Idris-dev#331 , allowing a segfault on returning a
null pointer string is very bad. This requires that either the
value is present or it returns Nothing.
Merge pull request #489 from gabemc/issue/331

Issue #331: Modifying the System.getEnv function to return IO (Maybe String)