Handle command line option arguments larger than 2^31.

Handle command line option arguments larger than 2^31.
This allows e.g., splitting into files of size 2GB and larger,
and running split --lines=N with N=2^31 or more.
But for --line-bytes=N, the restriction that N <= SIZE_MAX
remains (for now), due to the way it is implemented.

Include "inttostr.h".
(bytes_split, lines_split, line_bytes_split, main):
Use uintmax_t, not size_t, for file sizes.
(main): Give a better diagnostic for option arguments == 0.
Use umaxtostr to print file sizes.