Update genkernel's default kernel configuration options to sane defaults for modern systems

[PaddyMac]

I've used genkernel for years to generate an initramfs, but until recently I'd never used it to generate a kernel. I recently used it, and one thing I discovered was kind of shocking. Despite the rapid pace of Linux kernel development, the default kernel configuration options apparently haven't been changed since 2011. And I suspect that even then some of the options were probably not reasonable defaults. As it is, there are several configuration options I had to change because they simply are not reasonable for a modern system. Apparently this aspect of genkernel has been completely neglected for almost a decade. Something like this probably requires a good bit of discussion regarding what sane defaults actually are, but at the very least, I think sane defaults would be those that don't require users to update their kernel configuration and recompile their kernel frequently as a result of error or warning messages from emerging software which says that the kernel configuration options are unsuitable.