shared/install: ignore unit symlinks when doing preset-all

Before, when interating over unit files during preset-all, behaviour was the
following:

- if we hit the real unit name first, presets were queried for that name, and
  that unit was enabled or disabled accordingly,

- if we hit an alias first (one of the symlinks chaining to the real unit), we
  checked the presets using the symlink name, and then proceeded to enable or
  disable the real unit.

E.g. for systemd-networkd.service we have the alias dbus-org.freedesktop.network1.service
(/usr/lib/systemd/system/dbus-org.freedesktop.network1.service), but the preset
is only for the systemd-networkd.service name. The service would be enabled or
disabled pseudorandomly depending on the order of iteration.

For "preset", behaviour was analogous: preset on the alias name disabled the
service (following the default disable policy), preset on the "real" name
applied the presets.

With the patch, for "preset" and "preset-all" we silently skip symlinks. This
gives mostly the right behaviour, with the limitation that presets on aliases
are ignored.  I think that presets on aliases are not that common (at least my
preset files on Fedora don't exhibit any such usage), and should not be
necessary, since whoever installs the preset can just refer to the real unit
file. It would be possible to overcome this limitation by gathering a list of
names of a unit first, and then checking whether *any* of the names matches the
presets list. That would require a significant redesign of the code, and be
a lot slower (since we would have to fully read all unit directories to preset
one unit) to so I'm not doing that for now.

With this patch, two properties are satisfied:
- preset-all and preset are idempotent, and the second and subsequent invocations
  do not produce any changes,
- preset-all and preset for a specific name produce the same state for that unit.

Fixes #3616.

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