[Libguestfs] [PATCH nbdkit 0/2] common: Add checked-overflow macros

In common/vector/vector.c use GCC/Clang built-in overflow operators. The first patch is a neutral change which adds comments. The second patch is the actual change. Add a new header "checked-overflow.h" which has the purpose of isolating the use of the built-ins to one file (in case we need to add a new compiler later). Then use this in generic_vector_reserve. I tested this with GCC 11 and Clang 13. I verified by disassembly that "jo" (jump overflow) / "jno" is used where it was not used previously, by both compilers. Rich.