cli: Adjust integer range check, replace magic numbers with constants

cli: Adjust integer range check, replace magic numbers with constants

The value (1ULL << 63) - 1), used for checking that integer values are
within range, actually mean INT64_MAX, so use that instead.

Also, the negative case is not quite right: the value -(INT64_MAX + 1),
-9223372036854775808, is within the range of a signed 64-bits number
(assuming two's complement), but is not accepted right now.  Adjust the
check so it accepts that value.

I tested this patch manually: we accept  -9223372036854775808 but reject
-9223372036854775809.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>