Replace "master/slave" terms in code

Replace "master/slave" terms in code

Replace some "master/slave" terminology in the code with the preferred
"primary/secondary" keywords. This also changes user output such as
log messages, and fixes a typo ("seconary") in cfg_test.c.

There are still some references to "master" and "slave" for various
reasons:

- The old syntax can still be used as a synonym.
- The master syntax is kept when it refers to master files and formats.
- This commit replaces mainly keywords that are local. If "master" or
  "slave" is used in for example a structure that is all over the
  place, it is considered out of scope for the moment.