Just calling `atoi` to convert strings to integers does not check for valid integers very thoroughly; in particular, it just ignores everything starting from the first non-numeral character.  Even a string like "foo" is fine and silently returns 0.

This meant that `bitcoin-tx` would not fail if such a string was passed in various places where an integer is expected (like the `locktime` or an input/output index); this means that it would, for instance, silently accept a typo and interpret it in an unexpected way.

In this change, we use `ParseInt64` for parsing strings to integers, which actually verifies that the full string is valid as number.  New tests in the `bitcoin-util-test` cover the new error paths.

This fixes #13599.

make check