Use rc_getline instead of assuming a fixed kernel cmdline length.

This is now requires as COMMAND_LINE_SIZE isn't exposed by kernel
headers anymore.
Fixes #177.

src/librc/rc.map

@@ -12,6 +12,7 @@ global:
 	rc_deptree_update_needed;
 	rc_environ_fd;
 	rc_find_pids;
+	rc_getline;
 	rc_newer_than;
 	rc_older_than;
 	rc_runlevel_exists;