Signal creates an auto-constraints

[dadgar]

This PR adds a constraint such that jobs requesting signals only go to machines/drivers that can handle them.

[schmichael]

LGTM

Should we strings.ToLower() signals at some point to ensure they're compared in a normalized fashion? (eg SIGUSR1 == sigusr1)

[diptanu]

My opinion is that this constraint is not needed - A user can use the kernel constraint to make a job run on an OS which has the signals that the job needs. This is just another way of targeting a platform which supports a certain POSIX API.

And if a user mistypes a SIGNAL name, I think it's fine for it to be a runtime error which is no different than passing a wrong argument to the program.

[github-actions]

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