FILEIO: make filename arguments const.
FILEIO: fix signed/unsigned mismatches.
FILEIO: fix out of range comparison.
FILEIO: fix missing return value.
FILEIO: fix return value, should be an error code instead of NULL.
FILEIO: fix const of format arg.

Note that add_dubldigits is only called after start_double, which sets
Formatarg.str to Formatarg.strbuf, so it can be safely cast back to a
char *.