Check memunit parsing for overflow in confparse

Before, when parsing memunits, if overflow occured it failed silently.
Use nowrap u64 math to detect overflow, compare to INT64_MAX and if
greater tell user and fail accordingly.

15000000.5 TB fails double check as it a greater floating number than
(double)INT64_MAX

8388608.1 TB passes double check because it falls in the same value as
(double)INT64_MAX (which is 2^63), but will fail the int check because
(uint64_t)d, which is 2^63, is strictly greater than 2^63-1 (INT64_MAX).

Fixes #30920
Signed-off-by: José M. Guisado <guigom@riseup.net>