opt: functions to show integer values with kMGTPE suffixes

opt: functions to show integer values with kMGTPE suffixes

As with the set_ functions, there are twelve permutations of integer size,
base, and signedness.  The supported sizes are int, long, and long long.

For example, this:

 char buf1[OPT_SHOW_LEN];
 char buf2[OPT_SHOW_LEN];
 unsigned i = 1024000;
 opt_show_uintval_bi(buf1, &i);
 opt_show_uintval_si(buf2, &i);

will put "1000k" in buf1, and "1024k" in buf2.

Unlike the opt_set_ functions, these use unsigned arithmetic for unsigned values.

(32 bit bug using sizeof(suffixes) instead of strlen(suffixes) fixed by Rusty)