The type of std.os.argv should be null terminated

[fancl20]

From https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html looks like it's a standard behaviour? Is there any reason not marking them null terminated?

[ifreund]

Making the type of std.os.argv [:null]?[*:0]u8 would certainly be possible but would have much worse ergonomics in many cases due to the need to check unwrap the optional arguments which cannot be null.

I propose adding std.c.argv for this use case, with the type [*:null]?[*:0]u8 or [:null]?[*:0]u8 to encode argc as well.