strv: fix STRV_FOREACH_PAIR macro definition
sd-booted: update sd_booted() check a bit

Previously we were testing whether /sys/fs/cgroup/systemd/ was a mount
point. This might be problematic however, when the cgroup trees are bind
mounted into a container from the host (which should be absolutely
valid), which might create the impression that the container was running
systemd, but only the host actually is.

Replace this by a check for the existance of the directory
/run/systemd/system/, which should work unconditionally, since /run can
never be a bind mount but *must* be a tmpfs on systemd systems, which is
flushed at boots. This means that data in /run always reflects
information about the current boot, and only of the local container,
which makes it the perfect choice for a check like this.

(As side effect this is nice to Ubuntu people who now use logind with
the systemd cgroup hierarchy, where the old sd_booted() check misdetects
systemd, even though they still run legacy Upstart.)
logind: explicitly create state directories during early initialization

Strictly speaking this isn't necessary for the /run/systemd/seats/
directory, since that is created anyway as the first seat is found, and
seat0 is always found. But let's be explicit here, and also create the
sessions/ and users/ directories, so that people can always install
inotify watches from very early on, even when nobody logged in yet.
nspawn: don't make assumptions about the size of pid_t