scripted-diff: Rename pindexBestHeader, fHavePruned

...to m_best_header and m_have_pruned

-BEGIN VERIFY SCRIPT-
find_regex="\bpindexBestHeader\b" \
    && git grep -l -E "$find_regex" -- src \
        | xargs sed -i -E "s@$find_regex@m_best_header@g"
find_regex="\bfHavePruned\b" \
    && git grep -l -E "$find_regex" -- src \
        | xargs sed -i -E "s@$find_regex@m_have_pruned@g"
-END VERIFY SCRIPT-

src/validation.h

@@ -881,7 +881,7 @@ public:
     std::set<CBlockIndex*> m_failed_blocks;
 
     /** Best header we've seen so far (used for getheaders queries' starting points). */
-    CBlockIndex* pindexBestHeader = nullptr;
+    CBlockIndex* m_best_header = nullptr;
 
     //! The total number of bytes available for us to use across all in-memory
     //! coins caches. This will be split somehow across chainstates.