This also
- merges basic/linux and shared/linux,
- moves BPF_JUMP_A() to basic/missing_bpf.h,
- copies from usrspace kernel headers directory generated by 'make headers',
rather than copying from kernel tree,
- copies const.h into our tree to reduce change in ethtool.h,
- copies auto_fs.h into our tree to reduce change in auto_dev-ioctl.h.
Now linux/in.h has better conflict detection with glibc's
netinet/in.h. So, let's import the headers.
Note that our code already have many workarounds for the conflict,
but in this commit does not drop them. Let's do that in the later
commits if this really helps.
