remote-curl.c: xcurl_off_t is not portable (on 32 bit platfoms)

remote-curl.c: xcurl_off_t is not portable (on 32 bit platfoms)

When  setting
DEVELOPER = 1
DEVOPTS = extra-all

"gcc (Raspbian 6.3.0-18+rpi1+deb9u1) 6.3.0 20170516" errors out with
"comparison is always false due to limited range of data type"
"[-Werror=type-limits]"

It turns out that the function xcurl_off_t() has 2 flavours:

- It gives a warning 32 bit systems, like Linux
- It takes the signed ssize_t as a paramter, but the only caller is using
  a size_t (which is typically unsigned these days)

The original motivation of this function is to make sure that sizes > 2GiB
are handled correctly. The curl documentation says:
"For any given platform/compiler curl_off_t must be typedef'ed to a 64-bit
 wide signed integral data type"
On a 32 bit system "size_t" can be promoted into a 64 bit signed value
without loss of data, and therefore we may see the
"comparison is always false" warning.
On a 64 bit system it may happen, at least in theory, that size_t is > 2^63,
and then the promotion from an unsigned "size_t" into a signed "curl_off_t"
may be a problem.

One solution to suppress a possible compiler warning could be to remove
the function xcurl_off_t().

However, to be on the very safe side, we keep it and improve it:

- The len parameter is changed from ssize_t to size_t
- A temporally variable "size" is used, promoted int uintmax_t and the compared
  with "maximum_signed_value_of_type(curl_off_t)".
  Thanks to Junio C Hamano for this hint.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>