Force "restrict" not to be used when compiling with xlc.

Rely on sizeof(typename) rather than sizeof(variable) in pqformat.h.

Provide overflow safe integer math inline functions.

Use new overflow aware integer operations.

Use C99 restrict via pg_restrict, rather than restrict directly.

Work around overly strict restrict checks by MSVC.

Rewrite strnlen replacement implementation from 8a241792f96.