Pull up following revision(s) (requested by ozaki-r in ticket #628):
sys/net/if_ethersubr.c: revision 1.250
sys/net/if_ethersubr.c: revision 1.251
sys/net/if_ethersubr.c: revision 1.252
sys/net/if_ethersubr.c: revision 1.248
Use kmem_alloc instead of kmem_intr_alloc in ether_addmulti

ether_addmulti is now not called in softint thanks to wqinput that
pulled input routines of ICMP out of softint.

style

Fix the net.ether.multicast sysctl. If there is no multicast address
don't kmem_alloc(0) (which panics the kernel), and if the number of
multicast addresses has decreased don't copyout uninitialized kernel
data.

Several fixes:
- Style and typos
- Use kmem_zalloc, in case there is a padding between the fields of
  the structures
- Use ETHER_ADDR_LEN instead of a hard-coded '6'
- kmem_alloc(KM_SLEEP) can't fail
- Simplify ether_aton_r
- Use mutex_obj_free, not to leak memory

(martin)