bootctl: rework/modernize "unlink" and add Varlink API for it

Among other things this changes tracking of the location of resources
during GC from using the BootEntrySource enum rather than a path, since
we have that and it is more efficient and easier to grok.

shell-completion/bash/bootctl

@@ -40,8 +40,10 @@ _bootctl() {
                       --dry-run'
         [ARG]='--esp-path --boot-path --root --image --image-policy --install-source
                --variables --random-seed --make-entry-directory --entry-token --json
-               --efi-boot-option-description --secure-boot-auto-enroll --private-key
-               --private-key-source --certificate --certificate-source'
+               --efi-boot-option-description --efi-boot-option-description-with-device
+               --secure-boot-auto-enroll --private-key
+               --private-key-source --certificate --certificate-source
+               --oldest'
     )
 
     if __contains_word "$prev" ${OPTS[ARG]}; then
@@ -67,7 +69,7 @@ _bootctl() {
             --install-source)
                 comps="image host auto"
                 ;;
-            --random-seed|--variables|--secure-boot-auto-enroll)
+            --random-seed|--variables|--secure-boot-auto-enroll|--oldest|--efi-boot-option-description-with-device)
                 comps="yes no"
                 ;;
             --json)
@@ -85,7 +87,7 @@ _bootctl() {
 
     local -A VERBS=(
         [STANDALONE]='help status install update remove is-installed random-seed list set-timeout set-timeout-oneshot cleanup'
-        [BOOTENTRY]='set-default set-oneshot set-sysfail unlink'
+        [BOOTENTRY]='set-default set-oneshot set-sysfail set-preferred unlink'
         [BOOLEAN]='reboot-to-firmware'
         [FILE]='kernel-identify kernel-inspect'
     )