Similar to the change for sendmsg(), create a pointer size independent
implementation of recvmsg() and let cloudabi32 and cloudabi64 call into
it. In case userspace requests one or more file descriptors, call
kern_recvit() in such a way that we get the control message headers in
an mbuf. Iterate over all of the headers and copy the file descriptors
to userspace.
Reduce the potential amount of code duplication between cloudabi32 and
cloudabi64 by creating a cloudabi_sock_recv() utility function. The
cloudabi32 and cloudabi64 modules will then only contain code to convert
the iovecs to the native pointer size.
In cloudabi_sock_recv(), we can now construct an SCM_RIGHTS cmsghdr in
an mbuf and pass that on to kern_sendit().
Essentially, this is a literal copy of the code in sys/compat/cloudabi64,
except that it now makes use of 32-bits datatypes and limits. In
sys/conf/files, we now need to take care to build the code in
sys/compat/cloudabi if either COMPAT_CLOUDABI32 or COMPAT_CLOUDABI64 is
turned on.
This change does not yet include any of the CPU dependent bits. Right
now I have implementations for running i386 binaries both on i386 and
x86-64, which I will send out for review separately.
