It turns out that set_charset() invokes build_iovec() which modifies

It turns out that set_charset() invokes build_iovec() which modifies
iov address internally through realloc(3).  However, since the function
parameter wasn't designed to allow the modified iov being passed back to
the caller, we end up feeding iov with several corrupted entries(depends on
how many arguments were pushed into iovec before set_charset()) to nmount(2).

This commit fixes this regression introduced in rev1.31 such that
mount_cd9660(8) with code page conversion option(-C) enabled works again.

Reviewed by:	rodrigc