CFG_sync() rewrites unchanged minuisettings.txt

[nborodikhin]

Issues
There are two related issues:

• CFG_sync() rewrites settings file, even settings that were not changed
• CFG_init() invokes CFG_sync()

That is not a very big problem for NextUI and its bundled binaries as they are always consistent among themselves. However it becomes a lot worse when a pak uses NextUI's api.c and config.c and link them statically: if such a pak built against an older version performs CFG_init() on a newer NextUI system, it would strip configuration flags unknown to the pak, reverting them to default values.

Even worse than that, if configuration flags changed semantics (e.g. RGB->RGBA conversion of 6.11), an older pak would repeatedly write back wrong values on every start, potentially moving a value out of reasonable range.

Proposal
Change an approach to configuration read and write:

• CFG_init(), CFG_getX() and CFG_setX() (it if is set the same value) should not write to the settings file
• when file is being written:
  • only explicitly changed settings should be written with new values
  • unchanged settings should be left as-is (having the same serialized values as before the write)
  • unknown values should be preserved