Fix handling of Machine Check Exceptions.

Closes #764.

If non-fatal MCE is seen, abrt will detect it as an oops
and alert user in a usual manner. When user opens this
abrt problem for reporting, he will see that "comment"
field is pre-filled with a text.
What it says depends on whether mcelog tool is installed.
If mcelog is installed, the text will say that hardware errors
were detected, and will show the tail of either /var/log/mcelog
or syslog.
Otherwise the text will say that hardware errors
were detected, but they can't be usefully diagnosed,
and user is strongly advised to install mcelog tool.

If fatal MCE is encountered, kernel always panics,
(abrt has no chance of catching the oops),
kdump kicks in, and then after reboot abrt says that new vmcore
is found. When user generates backtrace, he will see oops text
which starts with
"Machine Check Exception: BANK nnn ..." and (hopefully)
is already explanatory enough.

(Yes, it's weird that kernel shows human-readable error messages
on fatal MCEs but doesn't do that for non-fatal ones.
This makes fetching MCE info significantly different...
I wish kernel would show human-readable MCEs in both cases,
we wouldn't need mcelog then... oh well.)

In order to generate meaningful hash for MCE's,
oops hashing was extended for oopses without backtraces.

Since MCEs, unlike regular oopses, don't contain kernel version,
additional magic is added to extract kernel version
in vmcore event handling.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>