Load modules through kmod

Instead of forking processes, load modules through libkmod.

This adds an optional package dependency to kmod but can be disabled by
--disable-kmod option in ./configure. If --enable-kmod option is given
but no kmod is found, loading will happen by forking a process.

The startup time has decreased by %36 on our embedded mips product. The
measurement is done by cgroup cpu shares.

Signed-off-by: David Goulet <dgoulet@efficios.com>