Ensure that a valid payload is selected when the target changes.

[bwatters-r7]

This is part of the ongoing guidance with regard to default payloads.
If we are going to start asking people to remove default payloads, then we need to ensure that the payload gets updated to a valid payload when someone changes the target.
See this conversation:
#20940 (comment)

TL;DR:
User loads a module, and when the module loads, it selects a valid payload for the selected target. The user then changes from say an ARCH_X64 payload target that was selected when the module loaded to an ARCH_CMD target. The selected payload does not update, and the user gets no notification until they run the exploit and are told the payload is incompatible. The payload should update when the target changes.