syscall: implement nametomib for openbsd.

Move the existing darwin/freebsd specific nametomib implementation
into the respective operating system dependent files.

Provide a nametomib implementation for openbsd, which operates on a
sysctl MIB that has been pre-generated from the various system headers
by mksysctl_openbsd.pl.

R=rsc
CC=golang-dev
https://golang.org/cl/4935044