syscall: remove getsockname workaround for openbsd
syscall: remove getsockname workaround for openbsd

Remove the getsockname workaround for unix domain sockets on OpenBSD.
This was fixed in OpenBSD 5.2 and we now have a minimum requirement
for OpenBSD 5.4-current.

R=golang-codereviews, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/50960043

pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme