util: dedup code in callers of serviceFlagToStr()

Introduce `serviceFlagsToStr()` which hides the internals of the bitmask
and simplifies callers of `serviceFlagToStr()`.

src/qt/guiutil.cpp

@@ -755,12 +755,8 @@ QString formatServicesStr(quint64 mask)
 {
     QStringList strList;
 
-    for (int i = 0; i < 64; i++) {
-        uint64_t check = 1ull << i;
-        if (mask & check)
-        {
-            strList.append(QString::fromStdString(serviceFlagToStr(i)));
-        }
+    for (const auto& flag : serviceFlagsToStr(mask)) {
+        strList.append(QString::fromStdString(flag));
     }
 
     if (strList.size())