FN_REFLEN

Problem: All mysys path functions assume the destination buffer being
passed to them is FN_REFLEN bytes long. But not all guarded against
overwriting the buffer, and not all ensured that the destination buffer
was '\0'-terminated.

Solution: Ensure destination buffer is not overflowed, and makes sure it
is '\0'-terminated. Add documentation which makes assumptions and
guarantees explicit.

Change-Id: I25e50f311df42209812b32aa710cea4d74ef9cee