Use kmod library to load modules in init script

[katiewasnothere]

This PR allows for loading kernel modules in the init binary that is run during UVM boot. This is necessary for running certain distros of linux since many builtin modules have been moved out into their own .ko files. We use the same library, libkmod, that is used by programs like systemd and dracut to accomplish this.

We could alternatively manually call the __NR_finit_module syscall with each .ko file that we find to load the modules. However, some modules have dependencies on other modules being loaded first, which would require us to manually track the dependency graph of modules. When you run depmod in a system, a series of files such as modules.dep are created under /lib/modules/<uname -r>. These files contain information about the modules, such as their dependencies. However, the man page specifically mentions that these files should not be read or used by any additional utilities as they may be subject to change in the future. libkmod handles dependencies internally.

WARNING: libkmod does not allow for static compilation, so we have to remove the static ldflag for building the init binary. This is a breaking change with older versions of hcsshim, since now we MUST have certain files present in the UVM for the init to run. If those files are not present, we will simply fail to start the UVM with an unintuitive error message.

Future work:

• Supporting allowlists and denylists to better control what should and shouldn't be loaded at boot time.