std: clear errno before readdir, then check it (solaris)

std: clear errno before readdir, then check it (solaris)

A `NULL` from `readdir` could be the end of stream or an error.  The
only way to know is to check `errno`, so it must be set to a known value
first, like a 0 that POSIX will never use.

This patch adds `set_errno`, uses it to clear the value before calling
`readdir`, then checks it again after to see the reason for a `NULL`.