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:

• [x] 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
• [x] modify section 4.3 for the changes of 4.2
• [x] remove -Dsplit-usr=true in systemd
• [x] 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

[Xi Ruoyao]

Initial changes are staged in branch xry111/usr-move now.

I think we can keep options like --libdir=/lib or --bindir=/bin there. Removing them for only systemd will cost too much work, and won't give significant improve (to readers) like removing explicit mv -vs.

I'll rebuild the system as a test, when GCC 11 is released.

And I hope I didn't introduce something breaking jhalfs...