Modify kernel module installation routine to respect --hostonly.

Modify kernel module installation routine to respect --hostonly.

If dracut was run with --hostonly, instmods will only load a module
into the initramfs if it is already loaded on the host machine.

This really trims the fat out of a --hostonly  generated initramfs, and
eliminates the need for the kernel-modules-loaded hook.

This patch also allows a module to flag that it should only load as
a dependency by exiting 255 instead of 0.  Currently, only the network module
uses this functionality.