syscall: add SetsockoptIpMreq
notes:
* due to Issue 1466 the Msghdr struct for
src/pkg/syscall/ztypes_darwin_386.go
src/pkg/syscall/ztypes_darwin_amd64.go
had to be edited after the godefs generation.
* ztypes_*.go files for linux, freebsd and darwin
have been prepared on the correct host OS and ARCH.
While the total increase is a dozen lines per file
the diff is larger due to a change to godefs,
http://code.google.com/p/go/source/detail?r=
c79e30afe9c8
while has altered the names of Pad members which
causes gofmt to realign the affected structs
R=rsc, mikioh
CC=golang-dev
https://golang.org/cl/
4119053