modmachine: Implement physical memory access using /dev/mem (Linux, etc).
This requires root access. And on recent Linux kernels, with CONFIG_STRICT_DEVMEM option enabled, only address ranges listed in /proc/iomem can be accessed. The above compiled-time option can be however overriden with boot-time option "iomem=relaxed". This also removed separate read/write paths - there unlikely would be a case when they're different.