Master & Relay Log Info: expand init_intvar_from_file from atoi to atoll

MDEV-25674's Master_Retry_Count is an unsigned long, which is larger (in theory and Linux) than the int range of the info-reading API's init_intvar_from_file.

Its PR currently parses the number from a string itself.
For consistency and future-proofing, we should expand init_intvar_from_file's limit itself, at least to long long but ideally with an unsigned long long overload as well (as in MySQL's version).

We must assess if this change impacts other numeric fields.
It would be a positive impact if a previous field fell for this trap and was reading over-int values truncated or clamped, because that's a bug.