Install default configuration into $out/share/factory

By default systemd should read all its configuration from /etc. Therefor
we rely on -Dsysconfdir=/etc in meson as default value. Unfortunately
this would also lead to installation of systemd's own configuration
files to `/etc` whereas we are limited to /nix/store. To counter that
this commit introduces two new configuration variables `factoryconfdir`
and `factorypkgconfdir` to install systemd's own configuration into nix
store again, while having executables looking up files in /etc.

sd-bus: if we receive an invalid dbus message, ignore and proceeed

dbus-daemon might have a slightly different idea of what a valid msg is
than us (for example regarding valid msg and field sizes). Let's hence
try to proceed if we can and thus drop messages rather than fail the
connection if we fail to validate a message.

Hopefully the differences in what is considered valid are not visible
for real-life usecases, but are specific to exploit attempts only.

(cherry picked from commit 6d586a1)

systemd v241-rc2

systemd v241-rc1

journal-remote: set a limit on the number of fields in a message

Existing use of E2BIG is replaced with ENOBUFS (entry too long), and E2BIG is
reused for the new error condition (too many fields).

This matches the change done for systemd-journald, hence forming the second
part of the fix for CVE-2018-16865
(https://bugzilla.redhat.com/show_bug.cgi?id=1653861).

(cherry picked from commit ef4d6ab)

systemd 240

Merge remote-tracking branch 'origin/nixos-v239' into nixos-v239

systemd 239

systemd 238

systemd 237