Improve portability of config(8).

• Use strlen(dp->d_name) instead of the unportable dp->d_namlen. Rename i to len to make it slightly more descriptive and prevent negative indexing of the array.
• Replace index() by strchr().

This supposedly fixes compilation on GNU systems.

Submitted by: Robert Millan <rmh debian org> (original patch)
MFC after: 3 weeks