CLEANUP: assorted typo fixes in the code and comments

These are mostly comments in the code. A few error messages were fixed
and are of low enough importance not to deserve a backport. Some regtests
were also fixed.
diff --git a/include/common/ist.h b/include/common/ist.h
index b19facd..68c83ff 100644
--- a/include/common/ist.h
+++ b/include/common/ist.h
@@ -722,7 +722,7 @@
 }
 
 /*
- * looks for the first occurence of <chr> in string <ist> and returns a shorter
+ * looks for the first occurrence of <chr> in string <ist> and returns a shorter
  * ist if char is found.
  */
 static inline struct ist iststop(const struct ist ist, char chr)