linux kernel insights

Creating this as a PR for @unixed so that we can track comments.

- Download kernel headers, enter camb directory, and type 'make'
- New sysfs directory /sys/kernel/camb created with two files undearneath it:
syscall_addr_modified and text_segment_hash.

File `syscall_addr_modified` is either 1 or 0 representing whether the syscall function pointers were modified or not respectively.
File `text_segment_hash` is the current sha1 hash of the kernel's .text segment (excluding loaded modules)

The address range that camb currently hashes is subject to change because it's probably not comprehensive. However, it caught the rootkits that I've thrown at it, one of which is suterusu (https://github.com/mncoppola/suterusu).
I moved this header to `osquery/kernel/linux/hash.h`.