FYI: new openat-like function: mkdirat

[Roland McGrath]

I think that the Solaris *at functions were really primarily intended for
use with O_XATTR to get at "file attribute" magic pseudo-directories rather
than to optimize use of normal directories and files.  Probably mkdirat
doesn't make sense in Solaris attribute pseudo-directories.  But mkdirat is
as useful in general as any of those *at additions, so I'd say we might as
well have it.