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

src/util.c

@@ -500,7 +500,7 @@ ssize_t slurp(const char *path, char **buf) {
     size_t n = fread(*buf, 1, stbuf.st_size, f);
     fclose(f);
     if ((ssize_t)n != stbuf.st_size) {
-        ELOG("File \"%s\" could not be read entirely: got %zd, want %zd\n", path, n, stbuf.st_size);
+        ELOG("File \"%s\" could not be read entirely: got %zd, want %" PRIi64 "\n", path, n, (int64_t)stbuf.st_size);
         free(*buf);
         *buf = NULL;
         return -1;