prog: add an attribute for syscalls which should not be generated

Create a `no_generate` attribute to be used with syscalls that
`syzkaller` should not try to generate from scratch. In other words,
`syzkaller` will only use seeds of this call. This will be useful for
syscalls which are unlikely to be correctly generated.

In particular, prevent these syscalls from being included in the choice
table or from being considered as possible resource constructors.
Also add a test which will attempt to generate programs with a bias
towards `no_generate` syscalls, and flag up any that make it into result
programs. Currently there are no `no_generate` syscalls, but the next
commit will add some.

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