afpacket: Fix support for 32-bit x86 arch

[adriansr]

afpacket uses syscalls unix.SYS_GETSOCKOPT and unix.SYS_SETSOCKOPT when it needs to call [gs]etsockopt() passing a raw pointer. This doesn't work for 32-bit x86 platforms as there's no such syscall, resulting in:

setsockopt packet_rx_ring: function not implemented
unix.Syscall(unix.SYS_GETSOCKOPT[=366],...) = 38 // ENOSYS

The correct way to call [gs]etsockopt() in this platform is to use the SYS_SOCKETCALL syscall with the right call parameter.

This patch refactors the raw [gs]etsockopt() calls in afpacket to use unix.GetsockoptString and unix.SetsockoptString so that it relies on Go runtime to call the appropriate syscall.

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[adriansr]

@googlebot I signed it!

[adriansr]

@googlebot I signed it!

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[adriansr]

@notti can I get a review? Sorry for the ping :)