add support for shadow variables

This adds support for shadow variables, which allow you to add new
"shadow" fields to existing data structures.

To allow patches to call the shadow functions in the core module, I had
to add a funky hack to use --warn-unresolved-symbols when linking, which
allows the patched vmlinux to link with the missing symbols.  I also
added greps to the log file to ensure that only unresolved symbols to
kpatch_shadow_* are allowed.  We can remove this hack once the core
module gets moved into the kernel tree.

Fixes #314.
Merge pull request #380 from jpoimboe/shadow-variables

add support for shadow variables