systemd-sleep: add support for freeze and standby

systemd-sleep: add support for freeze and standby

A new config file /etc/systemd/sleep.conf is added.
It is parsed by systemd-sleep and logind. The strings written
to /sys/power/disk and /sys/power/state can be configured.
This allows people to use different modes of suspend on
systems with broken or special hardware.

Configuration is shared between systemd-sleep and logind
to enable logind to answer the question "can the system be
put to sleep" as correctly as possible without actually
invoking the action. If the user configured systemd-sleep
to only use 'freeze', but current kernel does not support it,
logind will properly report that the system cannot be put
to sleep.

https://bugs.freedesktop.org/show_bug.cgi?id=57793
https://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commit;h=7e73c5ae6e7991a6c01f6d096ff8afaef4458c36
http://lists.freedesktop.org/archives/systemd-devel/2013-February/009238.html

SYSTEM_CONFIG_FILE and USER_CONFIG_FILE defines were removed
since they were used in only a few places and with the
addition of /etc/systemd/sleep.conf it becomes easier to just
append the name of each file to the dir name.