FFF_MARK_FORMAT mishandles affixes

[anakimluke]

Bug 1:
Steps to reproduce:

1. cd into a non-empty directory.
2. Export the environment variable export FFF_MARK_FORMAT="foo%f".
3. Launch fff.
4. Press m.
5. A * suffix will appear on the selected item.

The same bug happens when using %fbar; but you get an unexpected whitespace prefix, instead.

Expected behavior was for the default affixes to be suppressed.

Bug 2:
Steps to reproduce:

1. Same setup as Bug 1, but use %f instead of foo%f.
2. Notice the addition of both default affixes to the selected item.

The expected behavior was for no affixes to be added at all; the color change and status bar update would alert the user of the selection made.