Fixed free() errors in AuthUser::~AuthUser()

Fixed free() errors in AuthUser::~AuthUser()

AuthUser::~AuthUser() was freeing memory in the middle of an allocated
buffer.  I think AuthUser needs a strdup'd copy of the username,
rather than keeping a pointer to the buffer read from the helper.

Also I found 'xfree((char *)username());' really odd.  This username()
method does not return a reference so it shouldn't be used as LHS
in the macro.