Add memory writer implementation for Linux kernel

[osandov]

/dev/kmem is gone since torvalds/linux@bbcd53c. Any reasonable production kernel has CONFIG_DEVMEM=n or CONFIG_STRICT_DEVMEM=y, so we can't use /dev/mem either. Most production configs have CONFIG_KGDB=n, so we probably can't use kgdb.

This leaves creating our own kernel module. The tricky part will be providing a way for drgn to write to arbitrary kernel memory without making a giant back door for rootkits. I think it'd be good enough if we allow you to write to kernel memory if and only if you'd have permission to load a kernel module yourself; if you can load a kernel module, you can write to arbitrary memory anyways. Kernel module signing will need some extra consideration.