support plan 9

[smasher164]

The mmap abstraction needs to be filled in for Plan 9. We should be able to use segattach to get a segment of memory from the OS.

long segattach(int attr, char *class, void *va, ulong len);

The returned long corresponds to the starting address of the segment. If it is -1, that means segattach failed. I'm slightly unclear on the supported attributes. Looking through the Plan 9 source, I see that the only attributes declared in lib9.h are SG_RONLY (read only) and SG_CEXEC (detach on exec). Call sites in the linuxemu tree suggest that we should pass in SG_CEXEC into attr?

Additionally, the class name that is used is "memory". The man page states that the system will pick an address if va is 0, and I assume that len is the number of bytes desired.

So essentially,

addr := segattach(SG_CEXEC, "memory", 0, size)
if addr == -1 {
    return nil, some error
}
return unsafe.Pointer(addr), nil

We should use segdetach to represent munmap, since it only takes the starting address returned by segattach.

x/sys/plan9 will let us make these syscalls. However testing this on travis will be slightly complicated. We'd probably have to run a VM inside Ubuntu.