The following commands and options now support the standard size

The following commands and options now support the standard size
suffixes kB, M, MB, G, GB, and so on for T, P, Y, Z, and Y:
head -c, head -n, od -j, od -N, od -S, split -b, split -C,
tail -c, tail -n.
* doc/coreutils.texi (od invocation, head invocation, tail invocation):
Document support for new size suffixes.
(head invocation, tail invocation):
Document that -n uses the same suffixes as -c.
(tail invocation): More-clearly document what leading "+" does.
* src/head.c (usage, string_to_integer): Support new suffixes.
* src/od.c (usage, main): Likewise.
* src/split.c (usage, main): Likewise.
* src/tail.c (usage, parse_options): Likewise.
Prompted by a patch from Evan Hunt.