libpq: issue clear error message for nested service files

Previously an odd error message was generated.  Nested service files are
not supported.

Report by David Johnston

src/interfaces/libpq/fe-connect.c

@@ -4061,6 +4061,16 @@ parseServiceFile(const char *serviceFile,
 				}
 				*val++ = '\0';
 
+				if (strcmp(key, "service") == 0)
+				{
+					printfPQExpBuffer(errorMessage,
+									  libpq_gettext("nested service specifications not supported in service file \"%s\", line %d\n"),
+									  serviceFile,
+									  linenr);
+					fclose(f);
+					return 3;
+				}
+
 				/*
 				 * Set the parameter --- but don't override any previous
 				 * explicit setting.