http: fix some printf format warnings

http: fix some printf format warnings

Commit f8117f55 ("http: use off_t to store partial file size",
02-11-2015) changed the type of some variables from long to off_t.
Unfortunately, the off_t type is not portable and can be represented
by several different actual types (even multiple types on the same
platform). This makes it difficult to print an off_t variable in
a platform independent way. As a result, this commit causes gcc to
issue some printf format warnings on a couple of different platforms.

In order to suppress the warnings, change the format specifier to use
the PRIuMAX macro and cast the off_t argument to uintmax_t. (See also
the http_opt_request_remainder() function, which uses the same
solution).

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Jeff King <peff@peff.net>