Improve the handling of size suffixes for -C.

Support upper-case suffixes (K/M/G) as well as lower-case suffixes.

Make sure nothing *follows* the suffix.

We don't need to check for the suffix before parsing the number; the
parsing routines stop if they see a non-digit character.

(cherry picked from commit 5ff873f0d277e10ad9deffa92b00b38cbfef4914)