analyze: generalize cat-config to apply to tmpfiles, presets, hwdb.d, etc.

analyze: generalize cat-config to apply to tmpfiles, presets, hwdb.d, etc.

Fixes #10256.

What works:

systemd-analyze cat-config systemd/system-preset
systemd-analyze cat-config systemd/user-preset
systemd-analyze cat-config tmpfiles.d
systemd-analyze cat-config sysusers.d
systemd-analyze cat-config systemd/sleep.conf
systemd-analyze cat-config systemd/user.conf
systemd-analyze cat-config systemd/system.conf
systemd-analyze cat-config udev/udev.conf
(and other .conf files)
systemd-analyze cat-config udev/rules.d
systemd-analyze cat-config environment.d
systemd-analyze cat-config environment

Directories may be specified with the trailing dash or not.

The caveat is that for user configuration, systemd and other tools also look
at ~/.config/. It would be nice to support this, but this patch doesn't.
"cat-config --user" is rejected, and we may allow it in the future and then
extend the search path with directories under ~/.config.

What doesn't work (and probably shouldn't because those files cannot be
meaningfully concatenated):

systemd-analyze cat-config systemd/system  (.service, .slice, .socket, ...)
systemd-analyze cat-config systemd/user
systemd-analyze cat-config systemd/network (.network, .link, and .dnssd)

The hardcoding of information about paths in this manner is a bit ugly, but
OTOH, it is not too onerous, and at least we have one place where all the
schemes are "documented" through code. It'll make us think twice before adding
yet another slightly different scheme.