file-logger: Set owner/group of log file

The file is usually opened/created by root, however, if user/group IDs
are configured and the configuration is reloaded, the file will be reopened
as configured user.  Like with UNIX sockets we only attempt to change
the user if we have CAP_CHOWN allowing a start as regular user.

We don't have chown() on Windows, so check for it.

src/libcharon/bus/listeners/file_logger.c

@@ -243,6 +243,25 @@ METHOD(file_logger_t, open_, void,
 				 this->filename, strerror(errno));
 			return;
 		}
+#ifdef HAVE_CHOWN
+		if (lib->caps->check(lib->caps, CAP_CHOWN))
+		{
+			if (chown(this->filename, lib->caps->get_uid(lib->caps),
+					  lib->caps->get_gid(lib->caps)) != 0)
+			{
+				DBG1(DBG_NET, "changing owner/group for '%s' failed: %s",
+					 this->filename, strerror(errno));
+			}
+		}
+		else
+		{
+			if (chown(this->filename, -1, lib->caps->get_gid(lib->caps)) != 0)
+			{
+				DBG1(DBG_NET, "changing group for '%s' failed: %s",
+					 this->filename, strerror(errno));
+			}
+		}
+#endif /* HAVE_CHOWN */
 #ifdef HAVE_SETLINEBUF
 		if (flush_line)
 		{