fix random setsockopt failure (EINVAL)

[Sepherosa Ziehau]

Hi all,

Following patch should fix random setsockopt failure (EINVAL) in HEAD:
http://leaf.dragonflybsd.org/~sephe/sockopt.diff

In addition to that:
- Don't do copyout in setsockopt; the user arg is const
- Always setup sopt (i.e. set sopt.sopt_val to NULL early)
- In sys_getsocktopt(), centralize valsize check

Please review/test it.

Best Regard,
sephe

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