Consolidate service flag bit-to-name conversion to a shared serviceFlagToStr function

Side effect: this results in the RPC showing unknown service bits as "UNKNOWN[n]" like the GUI.

Note that there is no common mask-to-vector<string> function because both GUI and RPC would need to iterate through it to convert to their desired target formats.

src/protocol.h

@@ -237,7 +237,7 @@ const std::vector<std::string> &getAllNetMessageTypes();
 
 /** nServices flags */
 enum ServiceFlags : uint64_t {
-    // NOTE: When adding here, be sure to update qt/guiutil.cpp's formatServicesStr too
+    // NOTE: When adding here, be sure to update serviceFlagToStr too
     // Nothing
     NODE_NONE = 0,
     // NODE_NETWORK means that the node is capable of serving the complete block chain. It is currently
@@ -268,6 +268,8 @@ enum ServiceFlags : uint64_t {
     // BIP process.
 };
 
+std::string serviceFlagToStr(uint64_t mask, int bit);
+
 /**
  * Gets the set of service flags which are "desirable" for a given peer.
  *