libudev: modernization

This brings udev logging style a bit closer to normal systemd convention.
libudev: do not accept invalid log levels

Invalid log levels lead to a assert failure later on.

https://bugs.freedesktop.org/show_bug.cgi?id=85657
Use log "level" instead of "priority"

The term "priority" is misleading because higher levels have lower
priority. "Level" is clearer and shorter.

This commit touches only the textual descriptions, not function and variable
names themselves. "Priority" is used in various command-line switches and
protocol constants, so completly getting rid of "priority" is hard.

I also left "priority" in various places where the clarity suffered
when it was removed.
man: describe all log levels in udevadm(8)

https://bugs.freedesktop.org/show_bug.cgi?id=85657
calendarspec: add constant for weekdays_bits
sd-dhcp6-client: fix off-by-two error in DUID length

The duid data passed by the caller does not include the DUID type,
but sd_dhcp6_client_set_duid() was treating it like it did.
core: improve error message when machine id is missing
bus: use STR_IN_SET
man: explain journalctl --flush correctly
systemd-journal-flush.service: remove "trigger" from description

This service is now synchronous, so "trigger" is misleading.
kernel-install/90-loaderentry.install: fix cmdline parsing

A recent commit (2f3a215) changed the parsing of /proc/cmdline to use a
shell array. Unfortunately, this introduced a bug: "read -ar line"
populates the shell variable $r, not $line. This breaks installation of
new loader entries:

  # kernel-install add 3.17.1-304.fc21.x86_64 \
      /boot/vmlinuz-3.17.1-304.fc21.x86_64
  Could not determine the kernel command line parameters.
  Please specify the kernel command line in /etc/kernel/cmdline!

This commit alters the read command to correctly populate the $line
array instead.
TODO
scsi_id: fix usage spelling

s/threat/treat/g
libudev: Use correct free function

FILE * wants cleanup_fclose().

Spotted by udev hwdb segfaulting in gnome-continuous' buildroot
construction.