Tidy up sys_setgroups and kern_setgroups.

- 'groups' initialization to NULL is always ovewrwriten before use, so plug it
- get rid of 'goto out'
- kern_setgroups's callers already validate ngrp, so only assert the condition
- ngrp  is an u_int, so 'ngrp < 1' is more readable as 'ngrp == 0'

No functional changes.