Add overflow-safe math inline functions for unsigned integers

Add overflow-safe math inline functions for unsigned integers

Similarly to the signed versions added in 4d6ad31, this adds a set of
inline functions for overflow checks with unsigned integers, including
uint16, uint32 and uint64.  This relies on compiler built-in overflow
checks by default if available.  The behavior of unsigned integers is
well-defined so the fallback implementations checks are simple for
additions and subtractions.  Multiplications avoid division-based checks
which are expensive if possible, still this can happen for uint64 if
128-bit integers are not available.

While on it, the code in common/int.h is reorganized to avoid too many
duplicated comments.  The new macros will be used in a follow-up patch.

All thanks to Andres Freund for the input provided.

Author: Fabien Coelho, Michael Paquier
Discussion: https://postgr.es/m/20190830073423.GB2354@paquier.xyz