(PUP-392) FFI ADSI GetComputerName /Add Group SID

 - Some existing tests were using Sys::Admin.get_login to get the
   current username.  Implement this same functionality in
   Puppet::Util::Windows::ADSI::User.current_user_name by making
   Win32 API call directly instead of relying on sys/admin.  Note that
   unlike GetComputerNameW, GetUserNameW returns a length
   including the trailing NULL.
 - Further, replace calls to Sys::Admin::get_user(name).sid and
   Sys::Admin::get_group(name).sid with their respective equivalents in
   the ADSI classes, namely ADSI::User.new(name).sid and
   ADSI::Group.new(name).sid.  Note that Sys::Admin returned a string
   while ADSI returns a SID object which must be converted to a string.
 - To support retrieving the ADSI Group sid, add a new class member
   and method identical to the one on ADSI User.
(PUP-392) Remove sys-admin gem

 - Update inline strings mentioning it as a dependency
Merge pull request #2787 from Iristyle/ticket/master/PUP-392-validate-gems

(PUP-392) Remove sys-admin gem