Optimize debuginfod-client cache lookup/cleanup a little

[Mark Wielaard <mark@klomp.org>]

Hi,

debuginfod-client would try to create the cache config files twice,
once through debuginfod_init_cache, which was always called before the
debuginfod_clean_cache check. Which called debuginfod_config_cache
which also tried to create the config files when they didn't exist
yet. debuginfod_config_cache however had a small bug that meant it
would not provide a valid struct stat if the config file didn't exist
yet. The first patch fixes that:

[PATCH 1/3] debuginfod: Make sure debuginfod_config_cache always returns valid stat

Then the second patch removes debuginfod_init_cache which saves two
stat calls (but introduces a new mkdir call).

[PATCH 2/3] debuginfod: Remove debuginfod_init_cache

Finally as soon as debuginfod_clean_cache commits to clean the cache
dir we immediately update the mtime of the interval config file so
other threads will not try to simultaniously also try to clean up the
cache dir. Because that is just duplicate work.

[PATCH 3/3] debuginfod: update mtime of interval_path as early as possible

Cheers,

Mark