systemd-analyze: support discrete images for 'verify' verb

systemd-analyze: support discrete images for 'verify' verb

Adding --image parameter for verify verb using the dissect image functionality

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Example Run:

I created a unit service file testrun.service with an invalid key-value pairing
(foo = bar) and a squashfs image run.raw to test the code.

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

maanya-goenka@debian:~/systemd (img-support)$ mksquashfs img/ run.raw
Parallel mksquashfs: Using 4 processors
Creating 4.0 filesystem on run.raw, block size 131072.
[==============================================================================================================================|] 6/6 100%

Exportable Squashfs 4.0 filesystem, gzip compressed, data block size 131072
        compressed data, compressed metadata, compressed fragments, compressed xattrs
        duplicates are removed
Filesystem size 0.60 Kbytes (0.00 Mbytes)
        52.32% of uncompressed filesystem size (1.14 Kbytes)
Inode table size 166 bytes (0.16 Kbytes)
        43.01% of uncompressed inode table size (386 bytes)
Directory table size 153 bytes (0.15 Kbytes)
        58.40% of uncompressed directory table size (262 bytes)
Number of duplicate files found 1
Number of inodes 12
Number of files 6
Number of fragments 1
Number of symbolic links  0
Number of device nodes 0
Number of fifo nodes 0
Number of socket nodes 0
Number of directories 6
Number of ids (unique uids + gids) 1
Number of uids 1
        maanya-goenka (1000)
Number of gids 1
        maanya-goenka (1000)
maanya-goenka@debian:~/systemd (img-support)$ sudo build/systemd-analyze verify --image=run.raw testrun.service
/tmp/.#systemd-analyzec71c7297a936b91c/usr/lib/systemd/system/testrun.service:2: Unknown key name 'foo' in section 'Unit', ignoring.
testrun.service: Failed to create testrun.service/start: Unit sysinit.target not found.

The 'Unit sysinit.target not found' error that we see here is due to recursive dependency searching during
unit loading and has been addressed in a different PR:
systemd-analyze: add option to return an error value when unit verification fails #20233