ABI stabilization

ABI stabilization (WIP)

A large part of growing the adoption of a kernel is to stabilize the ABI used. I would like to work on a stable ABI in the next few months, and a 1.0 version of the kernel and syscall crate. I am proposing the following for ABI stabilization:

Move most file operations to the fd

Change chmod(path, mode) to fchmod(fd, mode)
Add fchown(fd, owner, group)

Make `dup` less magical

Change dup(fd, buf) to dup(fd)
Change dup2(fd, newfd, buf) to dup2(fd, newfd, buf)
Add openat(fd, path, flags)

Use standard methods for memory management

Use fmap to implement physmap
Use funmap to implement physunmap
Use open to implement physalloc
Use close to implement physfree
Remove virtophys
Consider using fmap to implement brk

Cleanup other syscalls

Use open and openat to implement pipe2
Implement or remove link(old, new)