shared/install: also check for self-aliases during installation and ignore them

We had a check that was done in unit_file_resolve_symlink(). Let's move
the check to unit_validate_alias_symlink_or_warn(), which makes it available
to the code in install.c.

With this, unit_file_resolve_symlink() behaves almost the same. The warning
about "suspicious symlink" is done a bit later. I think this should be OK.

test/test-systemctl-enable.sh

@@ -328,8 +328,7 @@ test ! -h "$root/etc/systemd/system/services.target.wants/templ1@two.service"
 test ! -e "$root/etc/systemd/system/link4.service"
 cat >"$root/etc/systemd/system/link4.service" <<EOF
 [Install]
-# FIXME: self-alias should be ignored
-# Alias=link4.service
+Alias=link4.service
 Alias=link4@.service
 Alias=link4@inst.service
 Alias=link4alias.service
@@ -372,8 +371,7 @@ test ! -h "$root/etc/systemd/system/link4alias2.service"
 test ! -e "$root/etc/systemd/system/link5.service"
 cat >"$root/etc/systemd/system/link5.service" <<EOF
 [Install]
-# FIXME: self-alias should be ignored
-# Alias=link5.service
+Alias=link5.service
 Alias=link5alias.service
 Alias=link5alias2.service
 EOF