build: numfmt.c: avoid a shadowing warning

build: numfmt.c: avoid a shadowing warning

* src/numfmt.c (parse_field_arg): Rename parameter s/optarg/arg/,
to avoid shadowing getopt's global variable.
Otherwise, building on OS X, with --enable-gcc-warnings, I saw this:

  In file included from src/numfmt.c:19:0:
  src/numfmt.c: In function 'parse_field_arg':
  ./lib/config.h:3109:25: error: declaration of 'rpl_optarg' shadows\
    a global declaration [-Werror=shadow]