typedef FileName not const?

Friends,

along the lines of other similar emails from me of late,
I tried to avoid casting away const when using the FileName
typedef.  There are several calls where a (const char *) has to
be cast to (char *) due to FileName being typedef'd as
non-const.  But changing the typedef to const doesn't seem to
conflict with any code in the source tree.

Since this may be seen as an external API change, I kept
these changes in their own patch submission, so that it can
be rejected separately if need be.

Mark Dilger