Ensure target/source architectures are compatible during sysmerge/sysupgrade.
In both sysmerge(8) and sysupgrade(8), ensure the target system's architecture is compatible with the source/installer's architecture.
This entails checking the target (or, in the case of sysmerge, the 'source') system's /etc/machine
file and comparing it to the machine
value returned by uname(2).
Fixes #515 (closed).