isprint/isgraph not strictly do-able in Lua

[Reuben Thomas <rrt@...>]

The weasel words of the ISO C standard say:

"the term control character refers to a member of a locale-specific
set of characters that are not printing characters."

Therefore one cannot, sadly, deduce that string.match(s, "%C") is the
same as isprint.

(isgraph is the same as isprint, but is not true for the space character).

I plan to add isprint and isgraph to luaposix for this reason (in
common with some other useful bits of the C standard library that are
missing from the Lua standard libraries).

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