intprops: Add INT_ADD_OK etc. * doc/intprops.texi (Checking Integer Overflow): New section. * lib/intprops.h: From a suggestion by Bruno Haible in: https://lists.gnu.org/r/bug-gnulib/2020-12/msg00051.html (SAFE_INT_ADD, SAFE_INT_SUBTRACT, SAFE_INT_MULTIPLY): New macros.