When setting config keys, the program doesn't know whether the
key-to-be-modified exists on the Config struct. (Perhaps, with
reflection, it is possible to find the field). To allow callers to write
non-existent keys, the program would...

Before:
1) converts the in-memory *Config to a map
2) sets the key on the map, and
3) writes this map to disk.
4) Then, it converts this map back into an in-memory struct.

This commit swaps 3 and 4 so the map can be validated against the struct
before being written to disk. This prevents the bug identified in #740.