shared/install: when creating symlinks, keep existing relative symlinks

shared/install: when creating symlinks, keep existing relative symlinks

Running preset-all on a system installed from rpms or even created
using make install would remove and recreate a lot of symlinks, changing
relative to absolute symlinks. In general relative symlinks are nicer,
so there is no reason to change them, and those spurious changes were
obscuring more interesting stuff.

$ make install DESTDIR=/var/tmp/inst1

$ systemctl preset-all --root=/var/tmp/inst1
(before)
Removed /var/tmp/inst1/etc/systemd/system/network-online.target.wants/systemd-networkd-wait-online.service.
Created symlink /var/tmp/inst1/etc/systemd/system/ctrl-alt-del.target → /usr/lib/systemd/system/exit.target.
Removed /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/remote-fs.target.
Created symlink /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/remote-fs.target → /usr/lib/systemd/system/remote-fs.target.
Created symlink /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/machines.target → /usr/lib/systemd/system/machines.target.
Created symlink /var/tmp/inst1/etc/systemd/system/sockets.target.wants/systemd-journal-remote.socket → /usr/lib/systemd/system/systemd-journal-remote.socket.
Removed /var/tmp/inst1/etc/systemd/system/sockets.target.wants/systemd-networkd.socket.
Created symlink /var/tmp/inst1/etc/systemd/system/sockets.target.wants/systemd-networkd.socket → /usr/lib/systemd/system/systemd-networkd.socket.
Removed /var/tmp/inst1/etc/systemd/system/getty.target.wants/getty@tty1.service.
Created symlink /var/tmp/inst1/etc/systemd/system/getty.target.wants/getty@tty1.service → /usr/lib/systemd/system/getty@.service.
Created symlink /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/systemd-journal-upload.service → /usr/lib/systemd/system/systemd-journal-upload.service.
Removed /var/tmp/inst1/etc/systemd/system/sysinit.target.wants/systemd-timesyncd.service.
Created symlink /var/tmp/inst1/etc/systemd/system/sysinit.target.wants/systemd-timesyncd.service → /usr/lib/systemd/system/systemd-timesyncd.service.
Removed /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/systemd-resolved.service.
Created symlink /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/systemd-resolved.service → /usr/lib/systemd/system/systemd-resolved.service.
Removed /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/systemd-networkd.service.
Created symlink /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/systemd-networkd.service → /usr/lib/systemd/system/systemd-networkd.service.

(after)
Removed /var/tmp/inst1/etc/systemd/system/network-online.target.wants/systemd-networkd-wait-online.service.
Created symlink /var/tmp/inst1/etc/systemd/system/ctrl-alt-del.target → /usr/lib/systemd/system/exit.target.
Created symlink /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/machines.target → /usr/lib/systemd/system/machines.target.
Created symlink /var/tmp/inst1/etc/systemd/system/sockets.target.wants/systemd-journal-remote.socket → /usr/lib/systemd/system/systemd-journal-remote.socket.
Created symlink /var/tmp/inst1/etc/systemd/system/multi-user.target.wants/systemd-journal-upload.service → /usr/lib/systemd/system/systemd-journal-upload.service.