Add implementations for rename and truncate

They are not quite working yet though, because we need to get things
fixed up on the akaros side of things.
Compile in for akaros as well as plan9
Add the Wstat and Fwstat syscalls for akaros

NOTE: passing a []byte paramter to a syscall generated via the //sys
directive causes it to actually expand to 2 parameters under the hood
when making the actual Syscall6 call.  The first is the byte array, the
second is its length.  Any subsequent arguments follow these two.
Stop building akaros as part of file_posix.go

This means we have to move all those functions over into file_akaros.go
directly.  This is necessary to be able to use the wstat and fwsata
syscalls for their appropriate library functions.
Remove cgo calls to chmod and fchmod

These are now implemented directly with wstat