Implement userspace oom killer - kill_by_rss()

Using the kernel oom killer has a few problems:
1) It is disallowed by default (even for root) on Fedora 20.
   You have to first write "1" into /proc/sys/kernel/sysrq to enable the "f"
    trigger.
2) The Chrome web browser assigns a penalty of 300 onto its own tab renderer
   processes. On an 8GB RAM machine, this means 2400MB, and will lead to every
   tab being killed before the actual memory hog
   See https://code.google.com/p/chromium/issues/detail?id=333617 for more info

Because of these issues, kill_by_rss() is used instead by default.
Substract Shmem (tmpfs) from Cached to get the actual page cache memory

See http://www.freelists.org/post/procps/library-properly-handle-memory-used-by-tmpfs
Add periodic status output

Also, include git version in startup output
Revert "Do not parse the complete /proc/meminfo to save CPU time"

We need Shmem to subtract tmpfs

This reverts commit 775c19d154f553df85c4f832de51c0e3df1edc2e.
Call opendir("/proc") on startup

I have seen it fail with "Resource temporarily unavailable" during
oom

Also enable actual process killing
print everything important to stderr

The the periodic status output can be redirected to /dev/null if
wanted
Dry-run oom kill to make sure stack grows to maximum size

This tries to make sure we cannot run out of memory later
Remove sleep in handle_oom - it was delaying startup

And is not strictly neccessary. At worst, we may end up killing the
same process twice, which does no harm
Right-align numbers
Don't panic if a processes statm cannot be read

It may have just exited.
Rename from "earlyoomd" to "earlyoom"
Add systemd unit file, add install target to Makefile
Print out first status line on stderr, so it gets logged by systemd
Update gitignore
Add readme