analyze: add 'cat-config' verb

analyze: add 'cat-config' verb

This is used as 'systemd-analyze show-config systemd/logind.conf', which
will dump
   /etc/systemd/system/user@.service
   /etc/systemd/system/user@.service.d/*.conf
   /run/systemd/system/user@.service.d/*.conf
   /usr/local/lib/systemd/system/user@.service.d/*.conf
   /usr/lib/systemd/system/user@.service.d/*.conf

The idea is to make it easy to dump the configuration using the same locations
and order that systemd programs use themselves (including masking, in the right
order, etc.). This is the generic variant that works with any configuration
scheme that follows the same general rules:

$ systemd-analyze cat-config systemd/system.conf
$ systemd-analyze cat-config systemd/user.conf
$ systemd-analyze cat-config systemd/logind.conf
$ systemd-analyze cat-config systemd/sleep.conf
$ systemd-analyze cat-config systemd/journald.conf
$ systemd-analyze cat-config systemd/journal-remote.conf
$ systemd-analyze cat-config systemd/journal-upload.conf
$ systemd-analyze cat-config systemd/coredump.conf
$ systemd-analyze cat-config systemd/resolved.conf
$ systemd-analyze cat-config systemd/timesyncd.conf
$ systemd-analyze cat-config udev/udev.conf