All includes that were not absolutely necessary were removed because
checks.h happens to very often be part of dependency loops. A warning
was added about this in check-t.h. The fields, enums and structs were
a bit tidied because it's particularly tedious to find anything there.
It would make sense to split this in two or more files (at least
extract tcp-checks).
The file was renamed to the singular because it was one of the rare
exceptions to have an "s" appended to its name compared to the struct
name.
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