Extend database entry replacement rules

These rules were previously ambiguous such that the targets of whatis
entries depended on the order in which files were scanned.

* libdb/db_store.c (enum replace_action): Add.
(replace_if_necessary): Extend rules for greater stability.  We now also
take mtimes and pointers into account, as well as including a case for
when the new ID is weaker than the old one (there was already a case for
when it was stronger).
(dbstore): Use `replace_if_necessary` when considering replacing multi
keys.

1 files changed, 2 insertions, 0 deletions

diff --git a/NEWS.md b/NEWS.md
index 73c1f2d2..4440c5a1 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -21,6 +21,8 @@ Improvements:
  * Maintain multi keys in sorted order, improving database reproducibility.
  * Pick a more consistent name for the target of a whatis entry in the
    database.
+ * Extend rules for when to replace one database entry with another,
+   producing more stable behaviour.
 
 man-db 2.10.2 (17 March 2022)
 =============================