systemd-analyze: add --root option for 'verify' verb and allow path parsing

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Example Run:

foobar.service created below is a service unit file that has a non-existing key-value
pairing (foo = bar) and is thus, syntactically invalid.

maanya-goenka@debian:~/systemd (img-support)$ cat <<EOF>img/usr/lib/systemd/system/foobar.service
> [Unit]
> foo = bar
>
> [Service]
> ExecStart = /opt/script0.sh
> EOF

The failure to create foobar.service because of the recursive dependency searching and verification has been addressed
in a different PR: systemd-analyze: add option to return an error value when unit verification fails #20233

maanya-goenka@debian:~/systemd (img-support)$ sudo build/systemd-analyze verify --root=img/ foobar.service
/home/maanya-goenka/systemd/img/usr/lib/systemd/system/foobar.service:2: Unknown key name 'foo' in section 'Unit', ignoring.
foobar.service: Failed to create foobar.service/start: Unit sysinit.target not found.

shell-completion/bash/systemd-analyze

@@ -125,7 +125,7 @@ _systemd_analyze() {
 
     elif __contains_word "$verb" ${VERBS[VERIFY]}; then
         if [[ $cur = -* ]]; then
-            comps='--help --version --system --user --global --man=no --generators=yes'
+            comps='--help --version --system --user --global --man=no --generators=yes --root'
         else
             comps=$( compgen -A file -- "$cur" )
             compopt -o filenames