Allow requirements to be enforceable via lua code

[ngie-eign]

I was thinking about this a bit and I believe it makes sense for there to be a series of routines written in Lua to enforce requirements. This would allow OSes, like FreeBSD, to write their own custom rules/requirements, keeping Kyua "pure" of per-OS logic, like the concept of kernel features in FreeBSD (which aren't available in Linux, for instance).

As part of this effort, it might make more sense to have Kyua support local configurations, similar to /usr/local/etc/kyua/kyua.conf, by default.

CC: @asomers, @ocochard