More typo fixes across the tree

Previously I have made spelling changes as caught by Debian's lintian
tool, which has a short list of common misspellings that it tests for.
This time I decided to run codespell against the source and fix all the
obvious problems it came up with. It also uses a list of misspellings
but it's much larger than lintian's, but also has some false positives
so I went over the list by hand.

The command I used to do check the source was:
git ls-tree -rz --name-only HEAD | xargs -0 codespell -L minimise,errorstring

1 files changed, 1 insertions, 1 deletions

diff --git a/metadata_hub.h b/metadata_hub.h
index 784f662..47965d5 100644
--- a/metadata_hub.h
+++ b/metadata_hub.h
@@ -106,7 +106,7 @@ void metadata_hub_release_track_artwork(void);
 // these functions lock and unlock the read-write mutex on the metadata hub and run the watchers
 // afterwards
 void metadata_hub_modify_prolog(void);
-void metadata_hub_modify_epilog(int modified); // set to true if modifications occured, 0 otherwise
+void metadata_hub_modify_epilog(int modified); // set to true if modifications occurred, 0 otherwise
 
 // these are for safe reading
 void metadata_hub_read_prolog(void);