bootchart: use 'n/a' if PRETTY_NAME is not found

Spotted with coverity. If parsing both /etc/os-release and
/usr/lib/os-release fails then null would be passed on. The calls
to parse the two files are allowed to fail. A empty /etc may not
have had the /etc/os-release symlink restored yet and we just
try again in the loop. If for whatever reason that does not happen
then we now pass on 'n/a' instead of null.
journalctl: make --utc work everywhere

The --utc option was introduced by commit
9fd290443f5f99fca0dcd4216b1de70f7d3b8db1.
Howerver, the implementation was incomplete.