Fix Char as Bool in interfaces (Jeremy Rubin)
Pull request description:
In a few places in src/wallet/wallet.h, we use a char when semantically we want a bool.
This is kind of an issue because it means we can unserialize the same transaction with different fFromMe flags (as differing chars) and evaluate the following section in wallet/wallet.cpp
```c++
if (wtxIn.fFromMe && wtxIn.fFromMe != wtx.fFromMe)
{
wtx.fFromMe = wtxIn.fFromMe;
fUpdated = true;
}
```
incorrectly (triggering an fUpdated where both fFromMe values represent true, via different chars).
I don't think this is a vulnerability, but it's just a little messy and unsemantic, and could lead to issues with stored wtxIns not being findable in a map by their hash.
The serialize/unserialize code for bool internally uses a char, so it should be safe to make this substitution.
NOTE: Technically, this is a behavior change -- I haven't checked too closely that nowhere is depending on storing information in this char. Theoretically, this could break something because after this change a tx unserialized with such a char would preserve it's value, but now it is converted to a ~true~ canonical bool.
https://github.com/bitcoin/bitcoin/pull/16572/commits/2dbfb37b407ed23b517f507d78fb77334142dce5
---
Backport of Core PR16572