Use a temporary buffer in sys_setgroups for requests with <= XU_NGROUPS groups.

Submitted by:	Tiwei Bie <btw mail.ustc.edu.cn>
X-Additional: JuniorJobs project
MFC after:	2 weeks
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.
Add "rpm" and "formfactor" LUN options to match istgt functionality.

MFC after:	1 week
MFV r273688:

Update filesystems magic.  This fixes a regression introduced by
libmagic 5.19 which prevents file(1) to show more verbose information
about a MBR.

Submitted by:	madpilot
MFC after:	2 weeks