Additonal safeguard for the ʟᴘ64 memory model : prefer size_t over int when appropriate+check all malloc() calls+other fixes

[ytrezq]

I took care to not use size_t when the signing bit might be required (at least if I didn’t mistake).

This adds an additional (but probably unnecessary) safeguard. As well fix probable issues for 16 bits x86 machines (in the case they use an ᴏꜱ already supported by exim) . At least this doesn’t hurt (until all return values ofmalloc()are checked which is something I fixed too).

Only core lib functions and the variables that use them are covered by this change (definitely unnecessary for the others). But I can add them too.

If despite testing I did something wrong, _please notice me_ of course.

This version leaves code that let the exim daemon continue running while still handling allocation errors. I also replaced direct calls tofree()withstore_free()