Add AbsPathForConfigVal to consolidate datadir prefixing for path args

Most commandline/config args are interpreted as relative to datadir if
not passed absolute. Consolidate the logic for this normalization.

src/util.h

@@ -191,6 +191,16 @@ bool OpenDebugLog();
 void ShrinkDebugFile();
 void runCommand(const std::string& strCommand);
 
+/**
+ * Most paths passed as configuration arguments are treated as relative to
+ * the datadir if they are not absolute.
+ *
+ * @param path The path to be conditionally prefixed with datadir.
+ * @param net_specific Forwarded to GetDataDir().
+ * @return The normalized path.
+ */
+fs::path AbsPathForConfigVal(const fs::path& path, bool net_specific = true);
+
 inline bool IsSwitchChar(char c)
 {
 #ifdef WIN32