make /lib, /lib64, /bin, /sbin symlinks to their conterpart in /usr

[Xi Ruoyao]

Support of "split-usr" configuration is deprecated in systemd-248, and likely to be removed in systemd-249.

Fedora and Arch have already made the move.

Specific to LFS, the main advantage of the move is we can delete all mv /usr/lib/libfoo.so.* /lib && ln -sfv ../../lib/$(...) /usr/lib/libfoo.so stuff. The main disadvantage is we'll have to use an initramfs if /usr is somewhere on network (but I doubt if there is really someone using LFS/BLFS this way).

It's planned to make the move only for systemd, for now. The steps:

• [ ] rewrite section 4.2 and section 7.5. In section 4.2 we'll set up {/usr,}/{lib,lib64,bin,sbin}, and the references to them should be removed in 7.5. For consistency both sysv and systemd should be changed: the revisions share a same section 7.5, but section 4.2 would be different
• [ ] modify section 4.3 for the changes of 4.2
• [ ] remove -Dsplit-usr=true in systemd
• [ ] mark all mv /usr/lib/libfoo.so.* /lib ... commands with revision="sysv", in LFS
• [ ] build and test LFS
• [ ] mark all mv /usr/lib/libfoo.so.* /lib ... commands with revision="sysv", in BLFS