Fix compilation warnings on all Debian architectures. (#3007)

Fix compilation warnings on all Debian architectures. (#3007)

stbuf.st_size is of type off_t, which the standard defines as “extended signed
integral type”¹, and for which there is no correct printf format string. Hence,
we need to cast it into a hopefully-large-enough type (ugh) and use the
corresponding format string. In our case, int64_t should do it, as config files
really shouldn’t be anywhere close to those numbers.

① http://pubs.opengroup.org/onlinepubs/007908799/xsh/systypes.h.html