config-linux: Is `syscalls` really required?

[wking]

While reviewing our required/optional settings, I noticed that linux.seccomp.sycalls is required, but that we don't require an entry in the array. That means "syscalls": [] would be technically valid, and I'm pretty sure that's not what we want.

If it makes sense to have a seccomp property that does not need syscalls entries, then syscalls should be optional.

If it does not makes sense to have an empty/unset syscalls then it should be required and have a minimum length of one.

Before #657, syscalls was omitempty (and therefore more optional-feeling, although there was no real Markdown spec for seccomp before #706, so it's hard to know). My current impression is that the property should be optional (and get its old omitempty back), but I'm fine either way as long as we have a consistent position.

[wking]

syscalls' names property is in the same boat. If names is required, I think we need to also require it to have a length ≥ 1.