Mark nbd as inactive until connected (#2422)

Mark nbd as inactive until connected (#2422)

Currently, 99-systemd.rules.in contains a line for network block
devices, which mark them as inactive until the first change event, and
as active from then on forward. This is not correct. A network block
device can be connected or disconnected; this state is signalled by the
presence or absense of a "pid" file, which contains the PID of the
nbd client userspace process that started the connection.

Update the rules file so that it checks for the presence of that file to
decide what to set SYSTEMD_READY to.

Note that current kernels do issue a change event upon connecting the
device, but not yet upon disconnecting. While it's possible to wait
until that's been fixed, the behaviour of the rule with TEST!="pid" in
the absence of a proper uevent is exactly the same as the behaviour of
the old rule; so it should be safe to apply now.

Signed-off-by: Wouter Verhelst <w@uter.be>