portablectl: display sysext-specific fields

portablectl: display sysext-specific fields

The wrong fields were being displayed, if at all.
ID and VERSION_ID in sysexts are used for matching, they
don't identify the sysext itself. Parse the newly defined
fields and display them separately from the compatibility
fields.

Before:

Image:
        /home/bluca/git/systemd/base.raw
Portable Service:
        n/a
Operating System:
        Debian GNU/Linux 10 (buster)
Extension:
        /home/bluca/git/systemd/app0.raw
        Extension Scope:
                n/a
        Extension Compatibility Level:
                n/a
        Portable Service:
                n/a
        Portable Prefixes:
                n/a
        Operating System:
                n/a (debian 10)
Extension:
        /home/bluca/git/systemd/app1.raw
        Extension Scope:
                n/a
        Extension Compatibility Level:
                n/a
        Portable Service:
                n/a
        Portable Prefixes:
                n/a
        Operating System:
                n/a (debian 10)
Unit files:
        app0.service
        app1.service

After:

Image:
        /home/bluca/git/systemd/base.raw
Portable Service:
        n/a
Operating System:
        Debian GNU/Linux 10 (buster)
Extension:
        /home/bluca/git/systemd/app0.raw
        Extension Scope:
                n/a
        Extension Compatibility Level:
                n/a
        Extension Compatibility OS:
                debian
        Extension Compatibility OS Version:
                10
        Portable Service:
                n/a
        Portable Prefixes:
                n/a
        Extension Image:
                ID: app Version: 0
Extension:
        /home/bluca/git/systemd/app1.raw
        Extension Scope:
                n/a
        Extension Compatibility Level:
                n/a
        Extension Compatibility OS:
                debian
        Extension Compatibility OS Version:
                10
        Portable Service:
                n/a
        Portable Prefixes:
                n/a
        Extension Image:
                ID: app Version: 1
Unit files:
        app0.service
        app1.service