Add support for NetBSD

[iquiw]

Test passed with rustc 1.25.0 (84203cac6 2018-03-25) on NetBSD amd64.

$ cargo test                                                                                         
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs                                                       
     Running target/debug/deps/users-03c12e2236b742fc                                                                  
                                                                                                                       
running 17 tests                                                                                                       
test base::test::uid ... ok                                                                                            
test base::test::group_by_name ... ok                                                                                  
test base::test::user_by_name ... ok                                                                                   
test base::test::uid_for_username ... ok                                                                               
test base::test::user_info ... ok                                                                                      
test mock::test::gid ... ok                                                                                            
test base::test::username_for_uid_for_username ... ok                                                                  
test base::test::username ... ok                                                                                       
test mock::test::group_name ... ok                                                                                     
test mock::test::no_current_username ... ok                                                                            
test mock::test::no_gid ... ok                                                                                         
test mock::test::no_uid ... ok                                                                                         
test mock::test::no_username ... ok                                                                                    
test mock::test::current_username ... ok                                                                               
test mock::test::no_group_name ... ok                                                                                  
test mock::test::uid ... ok                                                                                            
test mock::test::username ... ok                                                                                       
                                                                                                                       
test result: ok. 17 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out                                            
                                                                                                                       
   Doc-tests users                                                                                                     
                                                                                                                       
running 6 tests                                                                                                        
test src/lib.rs -  (line 97) ... ok                                                                                    
test src/lib.rs -  (line 42) ... ok                                                                                    
test src/lib.rs -  (line 73) ... ok                                                                                    
test src/switch.rs - switch::switch_user_group (line 127) ... ok                                                       
test src/mock.rs - mock (line 19) ... ok                                                                               
test src/mock.rs - mock (line 41) ... ok                                                                               
                                                                                                                       
test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

[ogham]

Hey @iquiw — thanks for the PR. I’ve merged it, but I had to move your edits because the code was changed by an earlier PR. Would you mind giving it another test to see if it still works?

It's released as v0.7.0.

[iquiw]

@ogham Thank you! Test passed as previous. (Tested with Rust 1.25 as official 1.26 is not available due to similar problem as rust-lang/rustup#1410)