lposix: Use reentrant passwd and group lookup functions

[markj]

Summary

Avoid nasty surprises for code which calls posix.unistd.chmod() in a
loop while iterating over passwd or group entries.

Fix some style nits while here.

[imp]

The change is fine. Had a question about the code, but if itakes more than a sec to answer, then 'it was like that when I got here' is fine :)

libexec/flua/modules/lposix.c

112

So what happens if you pass in none or nil for this argument? You just get the -1 default?

[markj]

libexec/flua/modules/lposix.c

112

Yes, so chown("/foo/bar", "markj") doesn't change the group ownership.

If you want to avoid modifying the user ownership, you have to write chown("/foo/bar", -1, "imp") instead, though it's not immediately clear to me that that will work as implemented.

When dealing with lua integers, I tend to write something like:

lua_Integer i = lua_tointeger(L, 2);
if (i < 0 || i > UID_MAX)
    /* raise an error */
uid_t uid = (uid_t)i;
...

so that we don't silently truncate arguments.