The state of SystemOTA, as represented by ota.SystemState is loaded on
startup of the service and saved when the service exits.

As with ota.LoadState, the state file can be absent without causing
problems. To support "ephemeral" mode, the state is only saved if the
directory where it resides in already exists. If the directory
/var/lib/sysota does not exist, state is discarded and the service will
run without any state present.

Add SIGTERM to the list of signals handled gracefully. Before this we
were only handling SIGINT but systemd is actually sending SIGTERM, which
abruptly ends the process.

Add tests measuring syscalls used by SaveState, behavior of the service
when the state file is corrupted, when the state directory is on a
read-only file system and when the state directory is on a file system
with no disk space left.

The state is not used for anything yet.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>