REORG: include: move checks.h to haproxy/check{,-t}.h

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.
18 files changed