tmpfiles: create /etc/resolv.conf as link to networkd's version, if it doesn't exist

If /etc/resolv.conf doesn't exist it's better than nothing to make it
point to networkd's version.
units: bring systemd-tmpfiles-setup-dev.service closer to systemd-tmpfiles-setup.service

Among other things, order both services relative to
systemd-sysusers.service in the same direction.
mount: tell /bin/mount to never touch /etc/mtab

/etc/mtab should die die die. It's sad enough util-linux still contains
support for it, but we don't have to partake in that charade, so let's
turn this off.

This is in-line with the fact that since years we already have been
"tainting" systemd if we detect /etc/mtab not being a symlink...

Of course, util-linux is currently broken, and still touches /etc/mtab,
weven if we pass "--no-mtab" to it:

https://bugzilla.redhat.com/show_bug.cgi?id=1109367

But hey, let's hope that gets fixed quickly, even if total removal of
/etc/mtab support from util-linux might not happen so quickly...
mount: add new SloppyOptions= setting for mount units, mapping to mount(8)'s "-s" switch
update TODO