fix: Fix bookkeeping of raw files

fix: Fix bookkeeping of raw files

Ccache 4.8-4.8.2 notes the size and count of raw files (written to the
cache if hard link or file clone mode is enabled) to a level 2 stats
file. However, the new cleanup algorithm (also introduced in 4.8)
assumes that all size/count statistics are placed in a level 1 stats
file. After a "ccache -c", the raw files will be "rediscovered" and
noted in level 1 stats files, thus duplicating bookkeeping in level 2
stats files, resulting in a too high cache size figure in "ccache -s".

To fix this, ccache will now:

- Note size/count of raw files in the proper level 1 stats file.
- When storing an entry on a cache miss, detect if the corresponding
  level 2 stats file has non-zero size/count statistics and propagate
  them to the level 1 stats file if so.
- Zero out files/count statistics for level 2 stats file on "ccache -c".

This means that the reported cache size will gradually correct itself
automatically. Alternatively, the user can run "ccache -c" to fix all
counters immediately.

Fixes #1320.