lib: extract igt_core.c

Same game as with ioctl_wrappers.c.

To split was rather clean except for the static function
oom_adjust_for_doom. But that was a bug, the calls to it in the
various open helpers should simply be done at init and fork time.
Which was already the case, except for simple testcase.  So fix it up.

While at it also start with a small section header for the
documentation.

v2: Don't forget to update the xml template ...

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

docs/reference/intel-gpu-tools/intel-gpu-tools-docs.xml

@@ -17,6 +17,7 @@
     <title>Intel GPU Tools</title>
     <xi:include href="xml/debug.xml"/>
     <xi:include href="xml/drmtest.xml"/>
+    <xi:include href="xml/igt_core.xml"/>
     <xi:include href="xml/igt_debugfs.xml"/>
     <xi:include href="xml/igt_display.xml"/>
     <xi:include href="xml/igt_kms.xml"/>