shared/install: reuse the standard symlink verification subroutine

We save a few lines, but the important thing is that we don't have two
different implementations with slightly different rules used for enablement
and loading. Fixes #22000.

Tested with:
- the report in #22000, it now says:
$ SYSTEMD_LOG_LEVEL=debug systemctl --root=/ enable test.service
Suspicious symlink /etc/systemd/system/test.service→/etc/systemd/system/myown.d/test.service, treating as alias.
unit_file_resolve_symlink: self-alias: /etc/systemd/system/test.service → test.service, ignoring.
running_in_chroot(): Permission denied
Suspicious symlink /etc/systemd/system/test.service→/etc/systemd/system/myown.d/test.service, treating as alias.
unit_file_resolve_symlink: self-alias: /etc/systemd/system/test.service → test.service, ignoring.
Failed to enable unit, refusing to operate on linked unit file test.service

- a symlink to /dev/null:
...
unit_file_resolve_symlink: linked unit file: /etc/systemd/system/test3.service → /dev/null
Failed to enable unit, unit /etc/systemd/system/test3.service is masked.

- the same from the host:
...
unit_file_resolve_symlink: linked unit file: /var/lib/machines/rawhide/etc/systemd/system/test3.service → /var/lib/machines/rawhide/dev/null
Failed to enable unit, unit /var/lib/machines/rawhide/etc/systemd/system/test3.service is masked.

- through the manager:
$ sudo systemctl enable test.service
Failed to enable unit: Refusing to operate on alias name or linked unit file: test.service
$ sudo systemctl enable test3.service
Failed to enable unit: Unit file /etc/systemd/system/test3.service is masked.

As seen in the first example, the warning is repeated. This is because we call
the lookup logic twice: first for sysv-compat, and then again for real. I think
that since this is only for broken setups, and when sysv-compat is enabled, and
in an infrequent manual operation, at debug level, this is OK.

src/basic/unit-file.h

@@ -44,6 +44,16 @@ int unit_symlink_name_compatible(const char *symlink, const char *target, bool i
 int unit_validate_alias_symlink_and_warn(const char *filename, const char *target);
 
 bool lookup_paths_timestamp_hash_same(const LookupPaths *lp, uint64_t timestamp_hash, uint64_t *ret_new);
+
+int unit_file_resolve_symlink(
+                const char *root_dir,
+                char **search_path,
+                const char *dir,
+                int dirfd,
+                const char *filename,
+                bool resolve_destination_target,
+                char **ret_destination);
+
 int unit_file_build_name_map(
                 const LookupPaths *lp,
                 uint64_t *cache_timestamp_hash,