bootctl: unlink and cleanup functions

The unlink command removes an entry from the ESP including
referenced files that are not referenced in other entries. That is
useful eg to have multiple entries that use the same kernel with
different options.

The cleanup command removes all files that are not referenced by any
entry.

shell-completion/bash/bootctl

@@ -31,7 +31,7 @@ _bootctl() {
     local i verb comps
     local cur=${COMP_WORDS[COMP_CWORD]} prev=${COMP_WORDS[COMP_CWORD-1]}
     local -A OPTS=(
-        [STANDALONE]='-h --help -p --print-esp-path -x --print-boot-path --version --no-variables --no-pager --graceful'
+        [STANDALONE]='-h --help -p --print-esp-path -x --print-boot-path --version --no-variables --no-pager --graceful --dry-run'
         [ARG]='--esp-path --boot-path --make-machine-id-directory --root --image --install-source'
     )
 
@@ -67,8 +67,8 @@ _bootctl() {
 
     local -A VERBS=(
         # systemd-efi-options takes an argument, but it is free-form, so we cannot complete it
-        [STANDALONE]='help status install update remove is-installed random-seed systemd-efi-options list set-timeout set-timeout-oneshot'
-        [BOOTENTRY]='set-default set-oneshot'
+        [STANDALONE]='help status install update remove is-installed random-seed systemd-efi-options list set-timeout set-timeout-oneshot cleanup'
+        [BOOTENTRY]='set-default set-oneshot unlink'
         [BOOLEAN]='reboot-to-firmware'
     )